1. The problem statement, all variables and given/known data Prove that the set of all trigonometric polynomials with integer coefficients is countable. 2. Relevant equations [itex] t(x)= a+\sum a_ncos(nx)+ \sum b_n sin(nx) [/itex] the sum is over n and is from 1 to some natural number. 3. The attempt at a solution So basically we have to look at all the possible trig polynomials of all finite lengths. with some natural a out front. lets first look at the ones where n=1 so we have a+cos(1x)+bsin(x) How about we map these to the first prime number 2. since a can be anything, if a is 1 then this t(x) will go to 2 and if a is 2 then t(x) will go to 2^2 if a is three then t(x) goes to 2^3. Now for the sum from n=1 to 2 will map these to the next prime and do the same process as above with the constant out front. Is this the right idea. I am mapping these to prime numbers so we can get a unique mapping and we don't have to worry about sending 2 things to one thing.