If you assume, that something is wrong, mention, what you observe. This is better than letting the readers guess, what the problem is.
You have defined an, but use a later. For me this code is running, when I change "an" to "a":
a = [1,-1,0,1,1];
b0 = a(1);
rm = 3.125e-07;
for k = 1:5
b(k) = b0 + rm^k * a(k);
end
This creates the elements of the sum, but not the sum itself. Perhaps you want:
S = 0;
for k = 1:5
S = S + rm^k * a(k);
end
or simpler:
k = 1:5;
S = sum(rm.^k .* a);
