It appears that A081860(n) - sum_{k=1..n} A141419(n,k)^2 > 0, if and only if 2*n+1 is a prime. Is this true? Also, the differences are the sequence {0,0,0,1,0,0,14,0,0,71,0,13,...}, so is there some pattern in the nonzero terms? Ed Jeffery http://oeis.org/A081860 http://oeis.org/A141419