LECTURES ON HILBERT CUB E MANIFOLD S 3 (5) If A C X is a Z-set, X is an A N R and U C X is open, then A n U C U /$ a Z-$ef . Proof . (1) an d (2) are trivial . Fo r (3) le t U be an ope n cove r of X an d for eac h n le t fn: X -» X - A n be a map . It is possibl e to selec t the f n sufficientl y clos e to id so tha t the infinit e lef t compositio n f = r ^ f n f n - r * ' f 2 f 1 define s a ma p of X into X — A whic h is limite d by U. Sinc e X ca n be assigne d a complet e metri c ther e is no proble m in choosin g the f n so tha t this limi t define s a map , i.e . is continuous . (Jus t choos e the f n so tha t d(f n »id) 1/2 n .) Als o ther e is no proble m in choosin g the f n so tha t f is limite d by U. T o achiev e the requiremen t f(X O A = $ we observ e tha t by 4.1 belo w we ca n choos e f ^ so tha t f-j (X ) is closed . Thu s f2,f3# • • • c a n be chose n so clos e to id tha t f(X ) O A j = f . Inductivel y repeatin g this sam e ide a we ca n choos e the f n so tha t f{X ) O A n = $ , for al l n. Fo r (4) we not e tha t f:U-* U —A ma y be chose n sufficientl y clos e to id y so tha t f extend s by the identit y to a ma p f: X -» X — A . Thi s require s tha t f be chose n U-clos e to id y , wher e U is an ope n cove r of U whic h consist s of ope n set s whos e diameter s approac h zer o as the y ge t close r to X — U . T o prov e (5) first observ e tha t by (3) we ma y assum e A compac t in U . Choos e ope n set s U-j an d U 2 in X so tha t A C U | C C l ^ l ^ ) C U 2 C CI X IU 2 ) C U . If f: X - X - A is chose n sufficientl y clos e to i d x , the n f(O x (U 2 ) - U| ) is a subse t Off U - A an d the restrictio n f | CI X (U 2 ) - U-, : CI X (U 2 ) - U-, - U - A is nomotopi c to the inclusion , with the entir e homotop y takin g plac e in U — A . (Her e we invok e the fac t tha t "close " map s into an AN R (= U — A ) are homotopi c via a "small " homotopy. ) Usin g this homotop y we ca n easil y buil d a ma p g : C l x ( U 2 ) - U 1 - ^ U - A suc h tha t g|Bd x (U 2 ) = id an d g = f on Bd x (U-|) . Exten d g to a ma p g : U - * U — A by definin g

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