-Given any map http://dientuvietnam...etex.cgi?F:X->Z and any homotopy http://dientuvietnam...i?[0,1]->Z , such that http://dientuvietnam...metex.cgi?d(x,0)=F|_{Y} there exits a homotopy http://dientuvietnam...i?[0,1]->Z such that http://dientuvietnam...ex.cgi?D(x,0}=F and .
If X is an Euclidean neighborhood retracts (ENR) show that Y X has HEP if and only if Y is a closed neiborhood retract in X .
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