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# (a) Show that (Ui, φi) is a smooth atlas

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HOMEWORK MATH-GA 2350.001 DIFFERENTIAL GEOMETRY I (1, 2, 4 are due by September, 19, 2016, 3,5, 6 are due by September, 26, 2016)

1. Show that a topological manifold M is connected iff M is path-connected.

2. Let RPn be the n-dimensional projective space, with the atlas given by the following functions

φi :Ui →Rn,[x1, . . . xn+1]7→(x1

xi, . . . ,xi−1

xi ,xi+1

xi , . . .xn+1 xi ),

whereUi, i= 1, . . . n+ 1 are open subsets{[x1, . . . , xn+1], xi 6= 0}.

(a) Show that (Ui, φi) is a smooth atlas.

(b) Show that RP1 is diffeomorphic to S1.

(c) Let π : S2 → RP2 be a map that sends a point (x, y, z) to a unique line through this point. Show that π is smooth and that π is local diffeomorphism: for any pointp∈S2 there exists an open neighborhood U ⊂M such that πU :U →π(U) is a diffeomorphism on an open subset of RP2.

3. Let0< r≤m≤n. LetVr ⊂Mn×m(R)be the set of matrices of rankr. Show that Vr is a smooth submanifold ofMn×m(R) and compute its dimension.

4. Let M be a manifold of class Ck. Let A, B ⊂M be closed subsets such that A∩B =∅. Show that there is a functionf ∈Ck(M)with values in [0,1]and such thatf is identically0 onA and identically 1 onB.

5. Is product of two smooth manifolds with boundary a smooth manifold with boundary?

6. Let n > 0 be an integer and leth,i be the euclidean scalar product on Rn+1, and Sn := {x ∈ Rn+1,||x|| = 1}. If x ∈ Rn+1 \ 0, we denote by [x] the corresponding point ofRPn.

(a) Show that the mapf :Sn×Sn→Rdefined by(x, y)7→ hx, yiis smooth.

Find all points were it is a submersion.

(b) Let M ⊂ Sn ×Sn consists of orthogonal couples. Show that M is a smooth submanifold1 of Sn×Sn.

(c) Let M0 ⊂ RPn×RPn consists of couples of orthogonal lines (L, L0) of Rn+1. Show that M0 is a smooth submanifold ofRPn×RPn.

(d) LetE be the set of triples(x, x0, y)ofSn×Sn×Rn+1 such thathx, x0i= hx, yi=hx0, yi= 0 andπ:E →M be the map(x, x0, y)7→(x, x0). Show that π is a smooth vector bundle over M.

1A subsetM N of a smoothn-manifoldN is a smooth submanifold if for any pointpM there is a chart(U, φ)ofN atpsuch thatφ(UM)is a submanifold ofφ(U)Rn in the sense of one of the four equivalent definitions given during the lecture

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(e) Let E0 be the set of couples ([x], y) of RPn×Rn+1 such that hx, yi= 0 and π0 :E0 →RPn be the map ([x], y)7→ [x]. Show that π0 is a smooth vector bundle.

(f) Let E00 be the set of couples (([x], y),([x0], y0)) of E0 × E0 such that hx, x0i = 0 and y = y0 and let π00 : E00 → M be the map defined by (([x], y),([x0], y0)) 7→ ([x],[x0]). Show that π00 is a is a smooth vector bundle.

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