It looks to me as if no "-1" value is there in the first 10,000 values. Is that correct? Is there a proof that no such value exists? A047949 a(2) = a(3) = 0; for n >= 4, a(n) = largest m such that n-m and n+m are both primes, or -1 if no such m exists. What is a(n) asymptotically?