Math 190A, Fall 2022 Homework 6
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Math 190A, Fall 2022
Homework 6
Due: Friday, December 2, 2022 11:59PM via Gradescope
(late submissions allowed up until December 3, 2022 11:59PM with -25% penalty)
Solutions must be clearly presented. Incoherent or unclear solutions will lose points.
(1) Let X and Y be spaces and set }p| = X* I X and }q| = Y* I Y . Prove that (X Ⅱ Y)* (X* Ⅱ Y* )/~ where the only nontrivial relation is p ~ q .
[Recall that Ⅱ means “disjoint union”, and if A and B are spaces, then a subset U of A Ⅱ B is open if and only if U n A and U n B are open in A and B, respectively.]
(2) Let X = Z>0 be the set of positive integers with the discrete topology. (a) Prove that X is locally compact, Hausdorff, and not compact.
(b) Prove that X* is homeomorphic to the subspace }0| u }1/d \ d e Z>0| of R.
(3) Let Y be a Hausdorff space and let X C Y be a locally compact subspace such that X = Y . Prove that X is an open subset of Y . Hints at end.
(4) Let n > 1 be an integer. Recall that RPn = (Rn+1 I }0|)/~ where x ~ y if there exists λ e R I }0| such that x = λy . Write [x1 : . . . : xn+1] for the equivalence class of (x1 , . . . , xn+1). Our goal is to show that RPn is a compactification of Rn ; since it’s a bit lengthy this problem will be worth 40 points rather than the usual 20.
Define U C RPn to be the subset of equivalence classes of the form [x1 : . . . : xn+1] where xn+1 0 (this makes sense since whether or not xn+1 is 0 does not depend on the actual representative).
(a) Prove that the function g : U → Rn given by
g([x1 : . . . : xn+1]) = ╱ , . . . , 、
is well-defined (i.e., does not depend on the choice of representative for the
equivalence class) and is a homeomorphism. Hint at end.
(b) Prove that U = RPn .
(c) Finally, prove that RPn is Hausdorff .
[You may use that the restriction π\S亢 : Sn → RPn is a quotient map, i.e., U C RPn is open if and only if (π\S亢 )-1 (U) is open. This does not follow from definitions and requires a proof, but you can take it for granted for this problem.]
(d) When n = 1, explain why RP1 I U is a single point and explain how this implies that RP1 S1 .
HINTs
3: Hint 1: By Proposition 4.3.19 (taking U = X), each x e X has a neighborhood V C X which is open in X such that ClX (V) is compact. Prove that V is also open in Y (see next hint for more help).
Hint 2: Continuing from hint 1, V = X n W for some open set W in Y . Explain why each of the following equalities holds:
W C ClY (W) = ClY (V) = ClX (V) C X.
4a: To show that g is continuous: let = π -1 (U) where π : Rn+1 / }0| → RPn is the quotient map. Define f : → Rn by the same formula as g and show that f (x) = g(π(x)) for all x e . Now use an argument very similar to the proof of Proposition 2.4.5.
0PTIoNAL PRoBLEMs (poN'T TURN IN)
(5) How do you describe (X x Y)* in terms of X* and Y* ?
(6) Prove that CPn is a compactification of Cn and that CP1 S2 .
(7) Pick 0 < k < n. Prove that Grk (Rn ) is a compactification of Rn and that Grk (Cn ) is a compactification of Cn .
2022-12-03