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March 2nd, 2011, 02:02 AM   #1
Joined: Nov 2010

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Algebraic Topology

Does anyone have any help with these questions?

(1) Let S^1 = {z element of Complex no. : |z| = 1} and let f : S1 -> S1 be the map defined
by f(z) = z^3. What is the induced homomorphism f* : Pi1(S^1) -> Pi1(S^1)?

(2) Let X be a space and let A be a subspace of X. Let i : A -> X be the
inclusion map. True or false: The induced homomorphism i* : Pi1(A) -> Pi1(X)
must be injective.(Justify your answer by providing either a proof or a

(3) Let G be a finitely generated abelian group. Using the previous problem
(or otherwise), construct a space X whose fundamental group is isomorphic
to G.

(4) Construct an example of a space X such that Pi1(X) is a cyclic group of order
n, where n is some positive integer.
Turloughmack is offline  
March 3rd, 2011, 07:56 PM   #2
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Re: Algebraic Topology

Hi Turloughmack.

These are questions worth writing out good proofs for, but I can maybe give a couple hints (not that I am a topologist, but I have gone over this material before!).

For the first problem, you really just need to figure out what the cubing map does to an element of the unit circle. So, for z ? S¹, write

and apply the cubing map. This should be enough to tell you the homomorphism of fundamental groups.

For the second problem, you should think of some very simple examples first. I would take and . Now think about the corresponding fundamental groups and whether the assertion holds.

The third and fourth problems are very similar in spirit. Note that any cyclic group is of course abelian and finitely generated. Now, recall the Fundamental Theorem of (Finitely Generated) Abelian Groups: Every finitely generated abelian group is a product of cyclic groups. You use this fact, but the actual construction of a space with a given group requires a bit of work, and is not amenable to a very brief review.

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