First you have to define what values of n to calculate C_n for. I will use 0 to 10:
n = 0:10;
Now you can calculate C_n. First initialize C_n to be a vector the same size as n with all zeros:
C_n = zeros(size(n));
Then calculate the value of C_n where n is odd (the value of C_n where n is even is already 0 because that's how C_n was initialized, so we don't need to do anything else for that):
idx = mod(n,2) == 1; % logical index with value 'true' where n is odd and 'false' where n is even
C_n(idx) = 8/pi^2./n(idx).^2; % calculating and storing 8/pi^2/n^2 where idx is 'true', i.e., n is odd
Finally create the stem plot:
figure();
stem(n,C_n);
