I have simplified the problem, using the fact that the covariance of a sum equals the sum of the covariances. I now have A = B + C, where the errors in B and C are known. what is cov(B, C)?
I do have a series of channels that contain the number of radioactive counts within a small energy range. Since the occurence of radioactive decay is statistical, the error in the number of counts is simply the square of the number of counts. Each channel contains counts from two different...