# Math 817: Problem set 6

Instructions: Write up any four of the following problems (but you should try all of them, and in the end you should be sure you know how to do all of them). Your write ups are due Friday, October 8, 2004. Each problem is worth 10 points, 9 points for correctness and 1 point for communication. (Your goal is not only to give correct answers but to communicate your ideas well. Make sure you use good English, so proofread your solutions. Once you finish a solution, you should restructure awkward sentences, and strike out anything that is not needed in your approach to the problem.)
• (a) Let M be the matrix whose columns are (1, 2, -1, 0)t, (4, 8, -4, -3)t, (0, 1, 3, 4)t, and (2, 5, 1, 4)t. Find a basis for the subspace of R4 spanned by the columns of M (the so-called column space of M).
• (b) Let A be the matrix whose rows are (2, 1, 2, 3), and (1, 1, 3, 0). The set of solutions x to Ax = 0 is a subspace of R4 called the nullspace of A. Find a basis for the nullspace of A.
1. Let n be a positive integer, let F be a field and let W be a subspace of Fn. Let W' be the set of all v in Fn such that vtw is 0 for all w in W.
• (a) Show that W' is a subspace of Fn.
• (b) Show that W'' contains W.
2. The set of all symmetric real nxn matrices is a subspace W of the real vector space Mn(R) of all real nxn matrices. Exhibit a basis and determine the dimension of W.
3. Problem #5 from 3.4:
• (i) Prove that the set B = {(1,2,0)t, (2,1,2)t, (3,1,1)t} is a basis of R3.
• (ii) Find the coordinate vector of the vector v = (1,2,3)t with respect to B (ordered as given).
• (iii) Let B' = {(0,1,0)t, (1,0,1)t, (2,1,0)t}. Find the change of basis matrix P relating the old basis B to the new basis B'.
• (iv) For which primes p is B a basis of Fp3?
4. Problem #11 from 3.4: Let F = Fp.
• (a) Compute the order of SL2(F).
• (b) Compute the number of (ordered) bases of Fn, and the orders of GLn(F) and SLn(F).