Quote by Daverz
v_a is arbitrary, so if v is not identically zero, t_aa must be zero for all a (think of v_1=1, the rest 0, then the same for a=2, 3, ...). Actually, I think that does it, because there's no way t_aa=0 for all a unless t is identically zero.

First of all, v_a is not arbitraryit is a vector that is part of the problem statement.
Second, why could you not have something like:
v_a = (1,0,1,0,1,1,1,0,...)
t_aa = (0,1,0,1,0,0,0,1,...)
This is a case where v is not identially zero and t_aa is NOT zero for all a