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Retraction (topology)

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3192:. Moreover, a locally compact ANR has the homotopy type of a locally finite CW complex; and, by West, a compact ANR has the homotopy type of a finite CW complex. In this sense, ANRs avoid all the homotopy-theoretic pathologies of arbitrary topological spaces. For example, the 662: 3554:
that is strictly locally contractible but is not homotopy equivalent to a CW complex. It is not known whether a compact (or locally compact) metrizable space that is strictly locally contractible must be an
2671: 2633: 2388: 2147: 3200:(for all choices of base point) is a homotopy equivalence. Since ANRs include topological manifolds, Hilbert cube manifolds, Banach manifolds, and so on, these results apply to a large class of spaces. 3337: 312: 3276: 924: 2887:
is an ANR but not an AR (because it is not contractible). In infinite dimensions, Hanner's theorem implies that every Hilbert cube manifold as well as the (rather different, for example not
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Any topological space that deformation retracts to a point is contractible and vice versa. However, there exist contractible spaces that do not strongly deformation retract to a point.
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is an ANR. An arbitrary CW complex need not be metrizable, but every CW complex has the homotopy type of an ANR (which is metrizable, by definition).
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if such a retraction exists. For instance, any non-empty space retracts to a point in the obvious way (any constant map yields a retraction). If
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has been found to give the most satisfactory theory. For that reason, the notations AR and ANR by themselves are used in this article to mean
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is a deformation retraction if it is a retraction and its composition with the inclusion is homotopic to the identity map on
275: 3587: 3181:.) Borsuk also found a compact subset of the Hilbert cube that is locally contractible (as defined above) but not an ANR. 4026: 1892: 1247: 3222: 932: 879: 694:
A retract need not be a deformation retract. For instance, having a single point as a deformation retract of a space
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Deformation retraction is a particular case of homotopy equivalence. In fact, two spaces are homotopy equivalent
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Among all closed inclusions, cofibrations can be characterized as follows. The inclusion of a closed subspace
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If a subspace is a retract of a space, then the inclusion induces an injection between fundamental groups.
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be a class of topological spaces, closed under homeomorphisms and passage to closed subsets. Following
756:. In this formulation, a deformation retraction carries with it a homotopy between the identity map on 2858:
for metrizable spaces.) It follows that every topological manifold is an ANR. For example, the sphere
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metrizable space is an ANR if and only if it is locally contractible in this sense. For example, the
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that preserves the position of all points in that subspace. The subspace is then called a
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An equivalent definition of deformation retraction is the following. A continuous map
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a positive integer, together with the closed line segment connecting the origin with
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that is not homotopy equivalent to a CW complex. Thus there is a metrizable space
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is contractible and not an AR, it is also not an ANR. By Cauty's theorem above,
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Continuous, position-preserving mapping from a topological space into a subspace
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If, in the definition of a deformation retraction, we add the requirement that
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A metrizable space is an AR if and only if it is contractible and an ANR. By
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For example, the inclusion of a subcomplex in a CW complex is a cofibration.
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they are both homeomorphic to deformation retracts of a single larger space.
657:{\displaystyle F(x,0)=x,\quad F(x,1)\in A,\quad {\mbox{and}}\quad F(a,1)=a.} 3876: 3840: 4135: 4092:
Puppe, Dieter (1967), "Bemerkungen über die Erweiterung von Homotopien",
3454:(that is, a complete metric linear space). (By Dugundji's theorem above, 842: 668: 458: 31: 4107: 4077: 3451: 1893:
Brouwer fixed-point theorem § A proof using homology or cohomology
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consisting of closed line segments connecting the origin and the point
1881: 4069: 830:. In other words, a strong deformation retraction leaves points in 3117:
that is an ANR but not strictly locally contractible. (A space is
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subset of the real line that is not an ANR, since it is not even
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Note that the condition of being a strong deformation retract is
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This article incorporates material from Neighborhood retract on
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West, James (2004), "Absolute retracts", in Hart, K. P. (ed.),
2666:{\displaystyle \operatorname {ANR} \left({\mathcal {M}}\right)} 4054:(1959), "On spaces having the homotopy type of a CW-complex", 3979:(1999), "Absolute neighborhood retracts and shape theory", in 3858:"Un espace métrique linéaire qui n'est pas un rétracte absolu" 3857: 3821: 3766: 2628:{\displaystyle \operatorname {AR} \left({\mathcal {M}}\right)} 1250:
for maps to any space. This is one of the central concepts of
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holds for ANRs: a map of ANRs that induces an isomorphism on
2383:{\textstyle \operatorname {ANR} \left({\mathcal {C}}\right),} 2654: 2616: 2578: 2550: 2483: 2419: 2368: 2334: 2246: 2180: 2142:{\textstyle \operatorname {AR} \left({\mathcal {C}}\right),} 2127: 2093: 2039: 901: 1172:
consisting of the line segment connecting the origin with
838:, take this as the definition of deformation retraction.) 3332:{\textstyle \left(X,B\right)\rightarrow \left(Y,A\right)} 3822:"Une caractérisation des rétractes absolus de voisinage" 3351:
of any CW complex has the homotopy type of a CW complex.
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as strong deformation retraction one can choose the map
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be any compact metrizable space with a closed subspace
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have been considered in this definition, but the class
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fixed throughout the homotopy. (Some authors, such as
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Every ANR has the homotopy type of a CW complex, by
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than being a deformation retract. For instance, let
2735:or not) is an AR. More concretely, Euclidean space 3546: 3526: 3506: 3486: 3466: 3438: 3413: 3386: 3366: 3331: 3270: 3173: 3153: 3133: 3109: 3088:Counterexamples: Borsuk found a compact subset of 3057: 3031: 3011: 2991: 2971: 2951: 2931: 2878: 2837:" topological spaces. Among their properties are: 2822: 2792: 2759: 2719: 2695: 2665: 2627: 2585: 2557: 2530: 2510: 2490: 2466: 2446: 2426: 2402: 2382: 2341: 2313: 2293: 2273: 2253: 2229: 2208: 2187: 2162: 2141: 2100: 2071: 2046: 2019: 1999: 1979: 1959: 1935: 1915: 1846: 1814: 1779: 1747: 1695: 1660: 1615: 1574: 1548: 1508: 1479: 1438: 1388: 1340: 1196: 1156: 1123: 1087: 1047: 1004: 918: 867: 799: 744: 687:. A deformation retraction is a special case of a 656: 514: 465:Deformation retract and strong deformation retract 445: 418:. Conversely, given any idempotent continuous map 394: 306: 250: 207: 145: 4057:Transactions of the American Mathematical Society 3203:Many mapping spaces are ANRs. In particular, let 1374: 4173:Creative Commons Attribution/Share-Alike License 3649:Hu (1965), Corollary II.14.2 and Theorem II.3.1. 3271:{\textstyle \left(Y,A\right)^{\left(X,B\right)}} 1220:Cofibration and neighborhood deformation retract 919:{\textstyle \mathbb {R} ^{n+1}\backslash \{0\};} 3347:) is an ANR. It follows, for example, that the 2852:by ANRs is an ANR. (That is, being an ANR is a 1891:−1)-sphere, is not a retract of the ball. (See 1005:{\displaystyle F(x,t)=(1-t)x+t{x \over \|x\|}.} 251:{\displaystyle \iota \colon A\hookrightarrow X} 3730:Hu (1965), Theorem VII.3.1 and Remark VII.2.3. 3712:Fritsch & Piccinini (1990), Theorem 5.2.1. 3676:Fritsch & Piccinini (1990), Theorem 5.2.1. 3374:is an ANR if and only if every open subset of 2919:in the sense that for every open neighborhood 667:In other words, a deformation retraction is a 325:. Note that, by definition, a retraction maps 81:type of topological space. For example, every 3161:contains a contractible open neighborhood of 671:between a retraction and the identity map on 8: 993: 987: 910: 904: 3422:(meaning a topological vector space with a 3892:Fritsch, Rudolf; Piccinini, Renzo (1990), 3875: 3839: 3808:, Warsaw: Państwowe Wydawnictwo Naukowe, 3784: 3539: 3519: 3499: 3479: 3459: 3431: 3406: 3379: 3359: 3286: 3246: 3224: 3166: 3146: 3126: 3101: 3097: 3096: 3093: 3044: 3024: 3004: 2984: 2964: 2944: 2924: 2870: 2864: 2814: 2808: 2781: 2775: 2748: 2744: 2743: 2740: 2712: 2703:is an AR; more generally, every nonempty 2688: 2653: 2652: 2640: 2615: 2614: 2602: 2577: 2576: 2574: 2549: 2548: 2546: 2523: 2503: 2482: 2481: 2479: 2459: 2439: 2418: 2417: 2415: 2395: 2367: 2366: 2354: 2333: 2332: 2330: 2306: 2286: 2266: 2245: 2244: 2242: 2222: 2201: 2179: 2178: 2176: 2155: 2126: 2125: 2113: 2092: 2091: 2089: 2064: 2038: 2037: 2035: 2012: 1992: 1972: 1952: 1928: 1908: 1827: 1792: 1760: 1728: 1673: 1628: 1587: 1561: 1520: 1492: 1451: 1401: 1365: 1353: 1309: 1304:, meaning that there is a continuous map 1177: 1148: 1144: 1143: 1140: 1104: 1068: 1060: 1039: 1035: 1034: 1031: 981: 934: 889: 885: 884: 881: 859: 853: 771: 725: 619: 561: 477: 453:we obtain a retraction onto the image of 423: 375: 295: 277: 231: 185: 126: 3426:metric) that is not an AR. One can take 1212:but not a strong deformation retract of 402:is a retraction, then the composition ι∘ 3565: 62:is a mapping that captures the idea of 4019:A Concise Course in Algebraic Topology 3394:has the homotopy type of a CW complex. 2841:Every open subset of an ANR is an ANR. 321:with the inclusion is the identity of 89:of a very simple topological space, a 3748:Cauty (1994), Fund. Math. 146: 85–99. 3739:Cauty (1994), Fund. Math. 144: 11–22. 1389:{\textstyle A=u^{-1}\!\left(0\right)} 515:{\displaystyle F\colon X\times \to X} 7: 1987:is a retract of some open subset of 1439:{\textstyle H:X\times \rightarrow X} 1899:Absolute neighborhood retract (ANR) 1661:{\textstyle H\left(x,1\right)\in A} 1273:), then the image of a cofibration 876:is a strong deformation retract of 265:, a retraction is a continuous map 3584:Fundamentals of Algebraic Topology 2680:, every locally convex metrizable 25: 3604:Hatcher (2002), Proposition 4H.1. 3474:cannot be locally convex.) Since 3207:be an ANR with a closed subspace 2848:, a metrizable space that has an 2833:ANRs form a remarkable class of " 4132:Encyclopedia of General Topology 3964:, Wayne State University Press, 2979:, there is an open neighborhood 1715:One basic property of a retract 1298:neighborhood deformation retract 1292:is a cofibration if and only if 1157:{\displaystyle \mathbb {R} ^{2}} 1048:{\displaystyle \mathbb {R} ^{2}} 3894:Cellular Structures in Topology 3058:{\textstyle V\hookrightarrow U} 2217:is a closed subset of a space 1755:) is that every continuous map 1341:{\displaystyle u:X\rightarrow } 1258:is always injective, in fact a 626: 618: 590: 4171:, which is licensed under the 3703:Borsuk (1967), Theorem V.11.1. 3640:Hu (1965), Proposition II.7.2. 3307: 3049: 2760:{\textstyle \mathbb {R} ^{n},} 2586:{\displaystyle {\mathcal {M}}} 2558:{\displaystyle {\mathcal {C}}} 2491:{\displaystyle {\mathcal {C}}} 2454:is a closed subset of a space 2427:{\displaystyle {\mathcal {C}}} 2342:{\displaystyle {\mathcal {C}}} 2254:{\displaystyle {\mathcal {C}}} 2188:{\displaystyle {\mathcal {C}}} 2101:{\displaystyle {\mathcal {C}}} 2047:{\displaystyle {\mathcal {C}}} 1815:{\textstyle g:X\rightarrow Y,} 1803: 1771: 1739: 1684: 1678: 1607: 1595: 1537: 1525: 1468: 1456: 1430: 1427: 1415: 1335: 1323: 1320: 1191: 1179: 1118: 1106: 1082: 1062: 969: 957: 951: 939: 788: 776: 736: 642: 630: 606: 594: 578: 566: 506: 503: 491: 434: 386: 242: 196: 190: 146:{\displaystyle r\colon X\to A} 137: 1: 3622:Hatcher (2002), Exercise 0.6. 3588:Graduate Texts in Mathematics 3354:By Cauty, a metrizable space 3119:strictly locally contractible 3110:{\textstyle \mathbb {R} ^{3}} 2518:is a neighborhood retract of 2323:absolute neighborhood retract 1780:{\textstyle f:A\rightarrow Y} 828:strong deformation retraction 85:is an ANR. Every ANR has the 71:absolute neighborhood retract 3694:Borsuk (1967), section IV.4. 2058:(starting in 1931), a space 1236:of topological spaces is a ( 1208:is a deformation retract of 317:that is, the composition of 223:. Equivalently, denoting by 4027:University of Chicago Press 3658:Hu (1965), Theorem III.8.1. 3121:if every open neighborhood 2727:is an AR. For example, any 1787:has at least one extension 1248:homotopy extension property 110:be a topological space and 4206: 3933:Cambridge University Press 3898:Cambridge University Press 3685:Hu (1965), Theorem V.7.1. 1696:{\displaystyle u(x)<1} 66:a space into a subspace. 58:of the original space. A 3667:Mardešiċ (1999), p. 245. 3631:Mardešiċ (1999), p. 242. 3211:that is an ANR, and let 3039:such that the inclusion 2823:{\textstyle I^{\omega }} 2682:topological vector space 1549:{\displaystyle H(a,t)=a} 800:{\displaystyle F(a,t)=a} 118:. Then a continuous map 3863:Fundamenta Mathematicae 3827:Fundamenta Mathematicae 3786:10.4064/fm-17-1-152-170 3772:Fundamenta Mathematicae 2707:of such a vector space 1923:of a topological space 1847:{\textstyle g=f\circ r} 1509:{\displaystyle x\in X,} 1088:{\displaystyle (1/n,1)} 3877:10.4064/fm-146-1-85-99 3856:Cauty, Robert (1994), 3841:10.4064/fm-144-1-11-22 3820:Cauty, Robert (1994), 3548: 3528: 3508: 3488: 3468: 3440: 3415: 3388: 3368: 3333: 3272: 3175: 3155: 3135: 3111: 3059: 3033: 3013: 2993: 2973: 2953: 2933: 2880: 2824: 2794: 2761: 2721: 2697: 2667: 2629: 2587: 2559: 2532: 2512: 2492: 2468: 2448: 2428: 2404: 2384: 2343: 2315: 2295: 2275: 2255: 2231: 2210: 2189: 2164: 2143: 2102: 2073: 2048: 2021: 2001: 1981: 1961: 1937: 1917: 1848: 1816: 1781: 1749: 1697: 1662: 1617: 1616:{\displaystyle t\in ,} 1576: 1575:{\displaystyle a\in A} 1550: 1510: 1481: 1440: 1390: 1342: 1198: 1158: 1125: 1089: 1049: 1006: 920: 869: 801: 746: 658: 527:deformation retraction 516: 447: 446:{\textstyle s:X\to X,} 396: 308: 252: 209: 147: 64:continuously shrinking 60:deformation retraction 4095:Archiv der Mathematik 3613:Puppe (1967), Satz 1. 3582:Weintraub, Steven H. 3549: 3529: 3509: 3489: 3469: 3441: 3424:translation-invariant 3416: 3397:By Cauty, there is a 3389: 3369: 3341:compact-open topology 3334: 3273: 3176: 3156: 3136: 3112: 3060: 3034: 3014: 2994: 2974: 2954: 2934: 2902:Every locally finite 2881: 2825: 2795: 2762: 2722: 2698: 2668: 2630: 2588: 2560: 2533: 2513: 2493: 2469: 2449: 2429: 2405: 2385: 2344: 2316: 2296: 2276: 2256: 2232: 2211: 2190: 2165: 2144: 2103: 2074: 2049: 2022: 2002: 1982: 1962: 1938: 1918: 1872:No-retraction theorem 1849: 1817: 1782: 1750: 1748:{\textstyle r:X\to A} 1698: 1663: 1618: 1577: 1551: 1511: 1482: 1480:{\textstyle H(x,0)=x} 1441: 1391: 1343: 1199: 1197:{\displaystyle (0,1)} 1159: 1126: 1124:{\displaystyle (0,1)} 1090: 1050: 1007: 921: 870: 802: 747: 745:{\textstyle r:X\to A} 659: 517: 448: 397: 395:{\textstyle r:X\to A} 309: 253: 210: 148: 3991:, pp. 241–269, 3721:West (2004), p. 119. 3538: 3518: 3498: 3478: 3458: 3430: 3405: 3378: 3358: 3285: 3223: 3165: 3145: 3125: 3092: 3043: 3023: 3003: 2983: 2963: 2943: 2923: 2916:locally contractible 2863: 2807: 2774: 2739: 2711: 2687: 2639: 2601: 2573: 2545: 2522: 2502: 2478: 2458: 2438: 2414: 2394: 2353: 2329: 2305: 2285: 2265: 2241: 2221: 2200: 2175: 2154: 2112: 2088: 2063: 2034: 2011: 1991: 1971: 1951: 1945:neighborhood retract 1927: 1907: 1826: 1791: 1759: 1727: 1672: 1627: 1586: 1560: 1519: 1491: 1450: 1400: 1352: 1308: 1271:weak Hausdorff space 1176: 1139: 1103: 1059: 1030: 933: 880: 852: 770: 724: 689:homotopy equivalence 560: 476: 422: 410:continuous map from 374: 276: 230: 184: 125: 83:topological manifold 77:) is a particularly 3985:History of Topology 3767:"Sur les rétractes" 3514:has an open subset 3399:metric linear space 2793:{\textstyle I^{n},} 2729:normed vector space 1268:compactly generated 1266:is Hausdorff (or a 1168:be the subspace of 1026:be the subspace of 841:As an example, the 681:deformation retract 457:by restricting the 208:{\textstyle r(a)=a} 4108:10.1007/BF01899475 3962:Theory of Retracts 3928:Algebraic Topology 3806:Theory of Retracts 3544: 3524: 3504: 3484: 3464: 3436: 3411: 3384: 3364: 3329: 3268: 3171: 3151: 3131: 3107: 3071:finite-dimensional 3065:is homotopic to a 3055: 3029: 3009: 2989: 2969: 2949: 2929: 2879:{\textstyle S^{n}} 2876: 2820: 2790: 2757: 2717: 2693: 2663: 2625: 2583: 2555: 2528: 2508: 2488: 2464: 2444: 2424: 2400: 2380: 2339: 2311: 2291: 2271: 2251: 2227: 2206: 2185: 2160: 2139: 2098: 2069: 2044: 2017: 1997: 1977: 1957: 1933: 1913: 1844: 1812: 1777: 1745: 1693: 1658: 1613: 1572: 1546: 1506: 1477: 1436: 1386: 1338: 1194: 1154: 1133:Euclidean topology 1121: 1085: 1045: 1002: 916: 868:{\textstyle S^{n}} 865: 797: 742: 706:(and in fact that 654: 624: 512: 443: 392: 304: 248: 205: 143: 44:continuous mapping 18:Retract (topology) 3590:. Vol. 270. 3219:. Then the space 3194:Whitehead theorem 3083:locally connected 2893:Hilbert manifolds 2595:metrizable spaces 1885:-dimensional ball 1723:(with retraction 1262:to its image. If 997: 698:would imply that 623: 469:A continuous map 48:topological space 16:(Redirected from 4197: 4156: 4126: 4088: 4047: 4024: 4009: 3972: 3953: 3918: 3888: 3879: 3852: 3843: 3816: 3797: 3788: 3749: 3746: 3740: 3737: 3731: 3728: 3722: 3719: 3713: 3710: 3704: 3701: 3695: 3692: 3686: 3683: 3677: 3674: 3668: 3665: 3659: 3656: 3650: 3647: 3641: 3638: 3632: 3629: 3623: 3620: 3614: 3611: 3605: 3602: 3596: 3595: 3579: 3573: 3570: 3553: 3551: 3550: 3545: 3533: 3531: 3530: 3525: 3513: 3511: 3510: 3505: 3493: 3491: 3490: 3485: 3473: 3471: 3470: 3465: 3445: 3443: 3442: 3437: 3420: 3418: 3417: 3412: 3393: 3391: 3390: 3385: 3373: 3371: 3370: 3365: 3338: 3336: 3335: 3330: 3328: 3324: 3306: 3302: 3277: 3275: 3274: 3269: 3267: 3266: 3265: 3261: 3245: 3241: 3180: 3178: 3177: 3172: 3160: 3158: 3157: 3152: 3140: 3138: 3137: 3132: 3116: 3114: 3113: 3108: 3106: 3105: 3100: 3064: 3062: 3061: 3056: 3038: 3036: 3035: 3030: 3018: 3016: 3015: 3010: 2998: 2996: 2995: 2990: 2978: 2976: 2975: 2970: 2958: 2956: 2955: 2950: 2938: 2936: 2935: 2930: 2897:Banach manifolds 2885: 2883: 2882: 2877: 2875: 2874: 2829: 2827: 2826: 2821: 2819: 2818: 2799: 2797: 2796: 2791: 2786: 2785: 2766: 2764: 2763: 2758: 2753: 2752: 2747: 2726: 2724: 2723: 2718: 2702: 2700: 2699: 2694: 2672: 2670: 2669: 2664: 2662: 2658: 2657: 2634: 2632: 2631: 2626: 2624: 2620: 2619: 2592: 2590: 2589: 2584: 2582: 2581: 2564: 2562: 2561: 2556: 2554: 2553: 2541:Various classes 2537: 2535: 2534: 2529: 2517: 2515: 2514: 2509: 2497: 2495: 2494: 2489: 2487: 2486: 2473: 2471: 2470: 2465: 2453: 2451: 2450: 2445: 2433: 2431: 2430: 2425: 2423: 2422: 2409: 2407: 2406: 2401: 2389: 2387: 2386: 2381: 2376: 2372: 2371: 2348: 2346: 2345: 2340: 2338: 2337: 2320: 2318: 2317: 2312: 2300: 2298: 2297: 2292: 2281:is a retract of 2280: 2278: 2277: 2272: 2260: 2258: 2257: 2252: 2250: 2249: 2236: 2234: 2233: 2228: 2215: 2213: 2212: 2207: 2194: 2192: 2191: 2186: 2184: 2183: 2169: 2167: 2166: 2161: 2148: 2146: 2145: 2140: 2135: 2131: 2130: 2107: 2105: 2104: 2099: 2097: 2096: 2082:absolute retract 2078: 2076: 2075: 2070: 2053: 2051: 2050: 2045: 2043: 2042: 2026: 2024: 2023: 2018: 2006: 2004: 2003: 1998: 1986: 1984: 1983: 1978: 1966: 1964: 1963: 1958: 1942: 1940: 1939: 1934: 1922: 1920: 1919: 1914: 1903:A closed subset 1887:, that is, the ( 1853: 1851: 1850: 1845: 1821: 1819: 1818: 1813: 1786: 1784: 1783: 1778: 1754: 1752: 1751: 1746: 1702: 1700: 1699: 1694: 1667: 1665: 1664: 1659: 1651: 1647: 1622: 1620: 1619: 1614: 1581: 1579: 1578: 1573: 1555: 1553: 1552: 1547: 1515: 1513: 1512: 1507: 1486: 1484: 1483: 1478: 1445: 1443: 1442: 1437: 1395: 1393: 1392: 1387: 1385: 1373: 1372: 1347: 1345: 1344: 1339: 1254:. A cofibration 1203: 1201: 1200: 1195: 1163: 1161: 1160: 1155: 1153: 1152: 1147: 1130: 1128: 1127: 1122: 1094: 1092: 1091: 1086: 1072: 1054: 1052: 1051: 1046: 1044: 1043: 1038: 1011: 1009: 1008: 1003: 998: 996: 982: 925: 923: 922: 917: 900: 899: 888: 874: 872: 871: 866: 864: 863: 806: 804: 803: 798: 751: 749: 748: 743: 663: 661: 660: 655: 625: 621: 533:onto a subspace 521: 519: 518: 513: 452: 450: 449: 444: 401: 399: 398: 393: 313: 311: 310: 305: 300: 299: 257: 255: 254: 249: 214: 212: 211: 206: 152: 150: 149: 144: 21: 4205: 4204: 4200: 4199: 4198: 4196: 4195: 4194: 4180: 4179: 4163: 4146: 4129: 4091: 4070:10.2307/1993204 4050: 4037: 4022: 4012: 3999: 3975: 3956: 3943: 3921: 3908: 3891: 3855: 3819: 3800: 3761: 3758: 3753: 3752: 3747: 3743: 3738: 3734: 3729: 3725: 3720: 3716: 3711: 3707: 3702: 3698: 3693: 3689: 3684: 3680: 3675: 3671: 3666: 3662: 3657: 3653: 3648: 3644: 3639: 3635: 3630: 3626: 3621: 3617: 3612: 3608: 3603: 3599: 3581: 3580: 3576: 3571: 3567: 3562: 3536: 3535: 3516: 3515: 3496: 3495: 3476: 3475: 3456: 3455: 3428: 3427: 3403: 3402: 3376: 3375: 3356: 3355: 3314: 3310: 3292: 3288: 3283: 3282: 3251: 3247: 3231: 3227: 3226: 3221: 3220: 3198:homotopy groups 3163: 3162: 3143: 3142: 3123: 3122: 3095: 3090: 3089: 3041: 3040: 3021: 3020: 3001: 3000: 2981: 2980: 2961: 2960: 2941: 2940: 2921: 2920: 2889:locally compact 2866: 2861: 2860: 2810: 2805: 2804: 2777: 2772: 2771: 2742: 2737: 2736: 2709: 2708: 2685: 2684: 2648: 2637: 2636: 2610: 2599: 2598: 2571: 2570: 2543: 2542: 2520: 2519: 2500: 2499: 2476: 2475: 2456: 2455: 2436: 2435: 2412: 2411: 2392: 2391: 2362: 2351: 2350: 2327: 2326: 2303: 2302: 2283: 2282: 2263: 2262: 2239: 2238: 2219: 2218: 2198: 2197: 2173: 2172: 2152: 2151: 2121: 2110: 2109: 2086: 2085: 2061: 2060: 2032: 2031: 2009: 2008: 1989: 1988: 1969: 1968: 1949: 1948: 1925: 1924: 1905: 1904: 1901: 1874: 1824: 1823: 1789: 1788: 1757: 1756: 1725: 1724: 1712: 1670: 1669: 1637: 1633: 1625: 1624: 1584: 1583: 1558: 1557: 1517: 1516: 1489: 1488: 1448: 1447: 1398: 1397: 1396:and a homotopy 1375: 1361: 1350: 1349: 1306: 1305: 1252:homotopy theory 1222: 1174: 1173: 1142: 1137: 1136: 1101: 1100: 1057: 1056: 1033: 1028: 1027: 986: 931: 930: 883: 878: 877: 855: 850: 849: 768: 767: 722: 721: 675:. The subspace 558: 557: 474: 473: 467: 420: 419: 372: 371: 291: 274: 273: 228: 227: 182: 181: 123: 122: 104: 99: 28: 23: 22: 15: 12: 11: 5: 4203: 4201: 4193: 4192: 4182: 4181: 4178: 4177: 4162: 4161:External links 4159: 4158: 4157: 4144: 4127: 4089: 4064:(2): 272–280, 4048: 4035: 4010: 3997: 3977:Mardešić, Sibe 3973: 3954: 3941: 3923:Hatcher, Allen 3919: 3906: 3889: 3853: 3817: 3798: 3757: 3754: 3751: 3750: 3741: 3732: 3723: 3714: 3705: 3696: 3687: 3678: 3669: 3660: 3651: 3642: 3633: 3624: 3615: 3606: 3597: 3574: 3572:Borsuk (1931). 3564: 3563: 3561: 3558: 3557: 3556: 3547:{\textstyle U} 3543: 3527:{\textstyle U} 3523: 3507:{\textstyle V} 3503: 3487:{\textstyle V} 3483: 3467:{\textstyle V} 3463: 3439:{\textstyle V} 3435: 3414:{\textstyle V} 3410: 3395: 3387:{\textstyle X} 3383: 3367:{\textstyle X} 3363: 3352: 3327: 3323: 3320: 3317: 3313: 3309: 3305: 3301: 3298: 3295: 3291: 3264: 3260: 3257: 3254: 3250: 3244: 3240: 3237: 3234: 3230: 3201: 3182: 3174:{\textstyle x} 3170: 3154:{\textstyle x} 3150: 3141:of each point 3134:{\textstyle U} 3130: 3104: 3099: 3086: 3054: 3051: 3048: 3032:{\textstyle U} 3028: 3012:{\textstyle x} 3008: 2992:{\textstyle V} 2988: 2972:{\textstyle X} 2968: 2952:{\textstyle x} 2948: 2932:{\textstyle U} 2928: 2907: 2900: 2873: 2869: 2855:local property 2842: 2817: 2813: 2789: 2784: 2780: 2756: 2751: 2746: 2720:{\textstyle V} 2716: 2696:{\textstyle V} 2692: 2661: 2656: 2651: 2647: 2644: 2623: 2618: 2613: 2609: 2606: 2580: 2552: 2531:{\textstyle Y} 2527: 2511:{\textstyle X} 2507: 2485: 2467:{\textstyle Y} 2463: 2447:{\textstyle X} 2443: 2421: 2403:{\textstyle X} 2399: 2379: 2375: 2370: 2365: 2361: 2358: 2336: 2325:for the class 2314:{\textstyle X} 2310: 2294:{\textstyle Y} 2290: 2274:{\textstyle X} 2270: 2248: 2230:{\textstyle Y} 2226: 2209:{\textstyle X} 2205: 2182: 2163:{\textstyle X} 2159: 2138: 2134: 2129: 2124: 2120: 2117: 2095: 2084:for the class 2072:{\textstyle X} 2068: 2041: 2020:{\textstyle X} 2016: 2007:that contains 2000:{\textstyle Y} 1996: 1980:{\textstyle X} 1976: 1960:{\textstyle Y} 1956: 1936:{\textstyle Y} 1932: 1916:{\textstyle X} 1912: 1900: 1897: 1873: 1870: 1869: 1868: 1865: 1862:if and only if 1858: 1855: 1843: 1840: 1837: 1834: 1831: 1811: 1808: 1805: 1802: 1799: 1796: 1776: 1773: 1770: 1767: 1764: 1744: 1741: 1738: 1735: 1732: 1711: 1708: 1692: 1689: 1686: 1683: 1680: 1677: 1657: 1654: 1650: 1646: 1643: 1640: 1636: 1632: 1612: 1609: 1606: 1603: 1600: 1597: 1594: 1591: 1571: 1568: 1565: 1545: 1542: 1539: 1536: 1533: 1530: 1527: 1524: 1505: 1502: 1499: 1496: 1476: 1473: 1470: 1467: 1464: 1461: 1458: 1455: 1435: 1432: 1429: 1426: 1423: 1420: 1417: 1414: 1411: 1408: 1405: 1384: 1381: 1378: 1371: 1368: 1364: 1360: 1357: 1337: 1334: 1331: 1328: 1325: 1322: 1319: 1316: 1313: 1246:if it has the 1221: 1218: 1193: 1190: 1187: 1184: 1181: 1151: 1146: 1120: 1117: 1114: 1111: 1108: 1084: 1081: 1078: 1075: 1071: 1067: 1064: 1042: 1037: 1013: 1012: 1001: 995: 992: 989: 985: 980: 977: 974: 971: 968: 965: 962: 959: 956: 953: 950: 947: 944: 941: 938: 915: 912: 909: 906: 903: 898: 895: 892: 887: 862: 858: 808: 807: 796: 793: 790: 787: 784: 781: 778: 775: 741: 738: 735: 732: 729: 704:path connected 665: 664: 653: 650: 647: 644: 641: 638: 635: 632: 629: 617: 614: 611: 608: 605: 602: 599: 596: 593: 589: 586: 583: 580: 577: 574: 571: 568: 565: 537:if, for every 523: 522: 511: 508: 505: 502: 499: 496: 493: 490: 487: 484: 481: 466: 463: 442: 439: 436: 433: 430: 427: 391: 388: 385: 382: 379: 315: 314: 303: 298: 294: 290: 287: 284: 281: 259: 258: 247: 244: 241: 238: 235: 204: 201: 198: 195: 192: 189: 154: 153: 142: 139: 136: 133: 130: 114:a subspace of 103: 100: 98: 95: 34:, a branch of 26: 24: 14: 13: 10: 9: 6: 4: 3: 2: 4202: 4191: 4188: 4187: 4185: 4176: 4174: 4170: 4165: 4164: 4160: 4155: 4151: 4147: 4145:0-444-50355-2 4141: 4137: 4134:, Amsterdam: 4133: 4128: 4125: 4121: 4117: 4113: 4109: 4105: 4101: 4097: 4096: 4090: 4087: 4083: 4079: 4075: 4071: 4067: 4063: 4059: 4058: 4053: 4049: 4046: 4042: 4038: 4036:0-226-51182-0 4032: 4028: 4021: 4020: 4015: 4014:May, J. Peter 4011: 4008: 4004: 4000: 3998:0-444-82375-1 3994: 3990: 3989:North-Holland 3987:, Amsterdam: 3986: 3982: 3978: 3974: 3971: 3967: 3963: 3959: 3955: 3952: 3948: 3944: 3942:0-521-79540-0 3938: 3934: 3930: 3929: 3924: 3920: 3917: 3913: 3909: 3907:0-521-32784-9 3903: 3899: 3895: 3890: 3887: 3883: 3878: 3873: 3869: 3865: 3864: 3859: 3854: 3851: 3847: 3842: 3837: 3833: 3829: 3828: 3823: 3818: 3815: 3811: 3807: 3803: 3802:Borsuk, Karol 3799: 3796: 3792: 3787: 3782: 3778: 3774: 3773: 3768: 3764: 3763:Borsuk, Karol 3760: 3759: 3755: 3745: 3742: 3736: 3733: 3727: 3724: 3718: 3715: 3709: 3706: 3700: 3697: 3691: 3688: 3682: 3679: 3673: 3670: 3664: 3661: 3655: 3652: 3646: 3643: 3637: 3634: 3628: 3625: 3619: 3616: 3610: 3607: 3601: 3598: 3594:. p. 20. 3593: 3589: 3585: 3578: 3575: 3569: 3566: 3559: 3541: 3521: 3501: 3481: 3461: 3453: 3449: 3433: 3425: 3421: 3408: 3400: 3396: 3381: 3361: 3353: 3350: 3346: 3345:mapping space 3342: 3325: 3321: 3318: 3315: 3311: 3303: 3299: 3296: 3293: 3289: 3281: 3262: 3258: 3255: 3252: 3248: 3242: 3238: 3235: 3232: 3228: 3218: 3214: 3210: 3206: 3202: 3199: 3195: 3191: 3187: 3183: 3168: 3148: 3128: 3120: 3102: 3087: 3084: 3080: 3076: 3072: 3068: 3052: 3046: 3026: 3019:contained in 3006: 2986: 2966: 2946: 2926: 2918: 2917: 2912: 2908: 2905: 2901: 2898: 2894: 2890: 2886: 2871: 2867: 2857: 2856: 2851: 2847: 2843: 2840: 2839: 2838: 2836: 2831: 2815: 2811: 2803: 2787: 2782: 2778: 2770: 2754: 2749: 2734: 2730: 2714: 2706: 2705:convex subset 2690: 2683: 2679: 2674: 2659: 2649: 2645: 2642: 2621: 2611: 2607: 2604: 2596: 2568: 2567:normal spaces 2539: 2525: 2505: 2461: 2441: 2434:and whenever 2397: 2377: 2373: 2363: 2359: 2356: 2324: 2308: 2288: 2268: 2224: 2216: 2203: 2195:and whenever 2170: 2157: 2136: 2132: 2122: 2118: 2115: 2083: 2080:is called an 2079: 2066: 2057: 2028: 2014: 1994: 1974: 1954: 1946: 1930: 1910: 1898: 1896: 1894: 1890: 1886: 1884: 1879: 1871: 1866: 1863: 1859: 1856: 1841: 1838: 1835: 1832: 1829: 1809: 1806: 1800: 1797: 1794: 1774: 1768: 1765: 1762: 1742: 1736: 1733: 1730: 1722: 1718: 1714: 1713: 1709: 1707: 1704: 1690: 1687: 1681: 1675: 1655: 1652: 1648: 1644: 1641: 1638: 1634: 1630: 1610: 1604: 1601: 1598: 1592: 1589: 1569: 1566: 1563: 1543: 1540: 1534: 1531: 1528: 1522: 1503: 1500: 1497: 1494: 1474: 1471: 1465: 1462: 1459: 1453: 1433: 1424: 1421: 1418: 1412: 1409: 1406: 1403: 1382: 1379: 1376: 1369: 1366: 1362: 1358: 1355: 1332: 1329: 1326: 1317: 1314: 1311: 1303: 1299: 1295: 1291: 1287: 1282: 1280: 1277:is closed in 1276: 1272: 1269: 1265: 1261: 1260:homeomorphism 1257: 1253: 1249: 1245: 1244: 1239: 1235: 1231: 1227: 1219: 1217: 1215: 1211: 1207: 1188: 1185: 1182: 1171: 1167: 1149: 1134: 1115: 1112: 1109: 1098: 1079: 1076: 1073: 1069: 1065: 1040: 1025: 1021: 1020: 999: 990: 983: 978: 975: 972: 966: 963: 960: 954: 948: 945: 942: 936: 929: 928: 927: 913: 907: 896: 893: 890: 875: 860: 856: 847: 845: 839: 837: 833: 829: 825: 821: 817: 813: 794: 791: 785: 782: 779: 773: 766: 765: 764: 761: 759: 755: 739: 733: 730: 727: 719: 715: 713: 709: 705: 701: 697: 692: 690: 686: 682: 678: 674: 670: 651: 648: 645: 639: 636: 633: 627: 615: 612: 609: 603: 600: 597: 591: 587: 584: 581: 575: 572: 569: 563: 556: 555: 554: 552: 548: 544: 540: 536: 532: 528: 509: 500: 497: 494: 488: 485: 482: 479: 472: 471: 470: 464: 462: 460: 456: 440: 437: 431: 428: 425: 417: 413: 409: 405: 389: 383: 380: 377: 368: 366: 362: 361:closed subset 358: 354: 350: 346: 342: 338: 335:. A subspace 334: 331: 328: 324: 320: 301: 296: 292: 288: 285: 282: 279: 272: 271: 270: 268: 264: 245: 239: 236: 233: 226: 225: 224: 222: 218: 202: 199: 193: 187: 179: 175: 171: 167: 163: 159: 140: 134: 131: 128: 121: 120: 119: 117: 113: 109: 101: 96: 94: 92: 88: 87:homotopy type 84: 80: 76: 72: 67: 65: 61: 57: 53: 49: 45: 41: 37: 33: 19: 4166: 4131: 4099: 4093: 4061: 4055: 4052:Milnor, John 4018: 3984: 3981:James, I. M. 3961: 3958:Hu, Sze-Tsen 3927: 3893: 3867: 3861: 3831: 3825: 3805: 3776: 3770: 3744: 3735: 3726: 3717: 3708: 3699: 3690: 3681: 3672: 3663: 3654: 3645: 3636: 3627: 3618: 3609: 3600: 3583: 3577: 3568: 3401: 3216: 3212: 3208: 3204: 3118: 3067:constant map 2914: 2910: 2859: 2853: 2835:well-behaved 2832: 2802:Hilbert cube 2675: 2540: 2322: 2196: 2150: 2081: 2059: 2029: 1944: 1943:is called a 1902: 1888: 1882: 1875: 1720: 1716: 1705: 1301: 1297: 1293: 1289: 1285: 1283: 1278: 1274: 1263: 1255: 1241: 1233: 1229: 1225: 1223: 1213: 1209: 1205: 1169: 1165: 1096: 1023: 1016: 1014: 848: 843: 840: 831: 827: 826:is called a 823: 819: 815: 811: 809: 762: 760:and itself. 757: 753: 717: 716: 712:contractible 707: 699: 695: 693: 684: 680: 679:is called a 676: 672: 666: 550: 546: 542: 538: 534: 530: 526: 524: 468: 454: 415: 411: 403: 369: 364: 356: 348: 344: 340: 339:is called a 336: 332: 326: 322: 318: 316: 266: 260: 220: 216: 177: 174:identity map 169: 165: 157: 155: 115: 111: 107: 105: 79:well-behaved 74: 70: 68: 63: 59: 55: 39: 29: 3779:: 152–170, 3278:of maps of 2939:of a point 1288:in a space 1243:cofibration 529:of a space 180:; that is, 162:restriction 97:Definitions 36:mathematics 4169:PlanetMath 3795:0003.02701 3756:References 3349:loop space 3339:(with the 3075:Cantor set 2909:Every ANR 2904:CW complex 2850:open cover 2349:, written 2301:. A space 2108:, written 1710:Properties 1446:such that 1164:. Now let 408:idempotent 359:must be a 269:such that 158:retraction 91:CW complex 40:retraction 4124:120021003 4102:: 81–88, 3870:: 85–99, 3834:: 11–22, 3448:separable 3308:→ 3186:Whitehead 3050:↪ 2899:are ANRs. 2830:are ARs. 2816:ω 2769:unit cube 2646:⁡ 2608:⁡ 2360:⁡ 2119:⁡ 1839:∘ 1804:→ 1772:→ 1740:→ 1653:∈ 1593:∈ 1567:∈ 1498:∈ 1431:→ 1413:× 1367:− 1321:→ 1017:strictly 994:‖ 988:‖ 964:− 902:∖ 737:→ 610:∈ 507:→ 489:× 483:: 435:→ 387:→ 353:Hausdorff 286:ι 283:∘ 263:inclusion 243:↪ 237:: 234:ι 138:→ 132:: 4190:Topology 4184:Category 4136:Elsevier 4016:(1999), 3960:(1965), 3925:(2002), 3804:(1967), 3765:(1931), 3592:Springer 2800:and the 2733:complete 2678:Dugundji 2565:such as 1878:boundary 1556:for all 1487:for all 1238:Hurewicz 1019:stronger 814:in and 810:for all 669:homotopy 459:codomain 215:for all 52:subspace 32:topology 4154:2049453 4116:0206954 4086:0100267 4078:1993204 4045:1702278 4007:1674915 3983:(ed.), 3970:0181977 3951:1867354 3916:1074175 3886:1305261 3850:1271475 3814:0216473 3452:F-space 3450:and an 3343:on the 3079:compact 1880:of the 1822:namely 1204:. Then 846:-sphere 836:Hatcher 822:, then 355:, then 341:retract 172:is the 160:if the 102:Retract 56:retract 50:into a 46:from a 4152:  4142:  4122:  4114:  4084:  4076:  4043:  4033:  4005:  3995:  3968:  3949:  3939:  3914:  3904:  3884:  3848:  3812:  3793:  3446:to be 3190:Milnor 2846:Hanner 2410:is in 2321:is an 2171:is in 2056:Borsuk 1224:A map 406:is an 4120:S2CID 4074:JSTOR 4023:(PDF) 3560:Notes 3280:pairs 3077:is a 1348:with 1296:is a 718:Note: 525:is a 156:is a 42:is a 4140:ISBN 4031:ISBN 3993:ISBN 3937:ISBN 3902:ISBN 3555:ANR. 3188:and 3069:. A 2895:and 2767:the 2635:and 2030:Let 1876:The 1688:< 1623:and 1582:and 1095:for 545:and 330:onto 261:the 106:Let 38:, a 4104:doi 4066:doi 3872:doi 3868:146 3836:doi 3832:144 3791:Zbl 3781:doi 2999:of 2959:in 2913:is 2844:By 2643:ANR 2593:of 2474:in 2390:if 2357:ANR 2237:in 2149:if 1967:if 1947:of 1895:.) 1719:of 1668:if 1300:of 1135:on 818:in 714:). 710:is 702:is 683:of 622:and 549:in 541:in 414:to 370:If 363:of 351:is 343:of 219:in 176:on 168:to 164:of 75:ANR 69:An 30:In 4186:: 4150:MR 4148:, 4138:, 4118:, 4112:MR 4110:, 4100:18 4098:, 4082:MR 4080:, 4072:, 4062:90 4060:, 4041:MR 4039:, 4029:, 4025:, 4003:MR 4001:, 3966:MR 3947:MR 3945:, 3935:, 3931:, 3912:MR 3910:, 3900:, 3896:, 3882:MR 3880:, 3866:, 3860:, 3846:MR 3844:, 3830:, 3824:, 3810:MR 3789:, 3777:17 3775:, 3769:, 3586:. 2891:) 2673:. 2605:AR 2538:. 2498:, 2261:, 2116:AR 2027:. 1703:. 1281:. 1240:) 1232:→ 1228:: 1216:. 691:. 553:, 461:. 367:. 293:id 93:. 4175:. 4106:: 4068:: 3874:: 3838:: 3783:: 3542:U 3522:U 3502:V 3482:V 3462:V 3434:V 3409:V 3382:X 3362:X 3326:) 3322:A 3319:, 3316:Y 3312:( 3304:) 3300:B 3297:, 3294:X 3290:( 3263:) 3259:B 3256:, 3253:X 3249:( 3243:) 3239:A 3236:, 3233:Y 3229:( 3217:B 3213:X 3209:A 3205:Y 3169:x 3149:x 3129:U 3103:3 3098:R 3085:. 3053:U 3047:V 3027:U 3007:x 2987:V 2967:X 2947:x 2927:U 2911:X 2872:n 2868:S 2812:I 2788:, 2783:n 2779:I 2755:, 2750:n 2745:R 2731:( 2715:V 2691:V 2660:) 2655:M 2650:( 2622:) 2617:M 2612:( 2579:M 2551:C 2526:Y 2506:X 2484:C 2462:Y 2442:X 2420:C 2398:X 2378:, 2374:) 2369:C 2364:( 2335:C 2309:X 2289:Y 2269:X 2247:C 2225:Y 2204:X 2181:C 2158:X 2137:, 2133:) 2128:C 2123:( 2094:C 2067:X 2040:C 2015:X 1995:Y 1975:X 1955:Y 1931:Y 1911:X 1889:n 1883:n 1854:. 1842:r 1836:f 1833:= 1830:g 1810:, 1807:Y 1801:X 1798:: 1795:g 1775:Y 1769:A 1766:: 1763:f 1743:A 1737:X 1734:: 1731:r 1721:X 1717:A 1691:1 1685:) 1682:x 1679:( 1676:u 1656:A 1649:) 1645:1 1642:, 1639:x 1635:( 1631:H 1611:, 1608:] 1605:1 1602:, 1599:0 1596:[ 1590:t 1570:A 1564:a 1544:a 1541:= 1538:) 1535:t 1532:, 1529:a 1526:( 1523:H 1504:, 1501:X 1495:x 1475:x 1472:= 1469:) 1466:0 1463:, 1460:x 1457:( 1454:H 1434:X 1428:] 1425:1 1422:, 1419:0 1416:[ 1410:X 1407:: 1404:H 1383:) 1380:0 1377:( 1370:1 1363:u 1359:= 1356:A 1336:] 1333:1 1330:, 1327:0 1324:[ 1318:X 1315:: 1312:u 1302:X 1294:A 1290:X 1286:A 1279:X 1275:f 1264:X 1256:f 1234:X 1230:A 1226:f 1214:X 1210:X 1206:A 1192:) 1189:1 1186:, 1183:0 1180:( 1170:X 1166:A 1150:2 1145:R 1119:) 1116:1 1113:, 1110:0 1107:( 1097:n 1083:) 1080:1 1077:, 1074:n 1070:/ 1066:1 1063:( 1041:2 1036:R 1024:X 1000:. 991:x 984:x 979:t 976:+ 973:x 970:) 967:t 961:1 958:( 955:= 952:) 949:t 946:, 943:x 940:( 937:F 914:; 911:} 908:0 905:{ 897:1 894:+ 891:n 886:R 861:n 857:S 844:n 832:A 824:F 820:A 816:a 812:t 795:a 792:= 789:) 786:t 783:, 780:a 777:( 774:F 758:X 754:X 740:A 734:X 731:: 728:r 708:X 700:X 696:X 685:X 677:A 673:X 652:. 649:a 646:= 643:) 640:1 637:, 634:a 631:( 628:F 616:, 613:A 607:) 604:1 601:, 598:x 595:( 592:F 588:, 585:x 582:= 579:) 576:0 573:, 570:x 567:( 564:F 551:A 547:a 543:X 539:x 535:A 531:X 510:X 504:] 501:1 498:, 495:0 492:[ 486:X 480:F 455:s 441:, 438:X 432:X 429:: 426:s 416:X 412:X 404:r 390:A 384:X 381:: 378:r 365:X 357:A 349:X 345:X 337:A 333:A 327:X 323:A 319:r 302:, 297:A 289:= 280:r 267:r 246:X 240:A 221:A 217:a 203:a 200:= 197:) 194:a 191:( 188:r 178:A 170:A 166:r 141:A 135:X 129:r 116:X 112:A 108:X 73:( 20:)

Index

Retract (topology)
topology
mathematics
continuous mapping
topological space
subspace
well-behaved
topological manifold
homotopy type
CW complex
restriction
identity map
inclusion
onto
Hausdorff
closed subset
idempotent
codomain
homotopy
homotopy equivalence
path connected
contractible
Hatcher
n-sphere
stronger
Euclidean topology
Hurewicz
cofibration
homotopy extension property
homotopy theory

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