  For k greater than 1 and r different from 0, let pi^k_{2r}(x) denote the
number of prime pairs (p,p^k+2r) with p not exceeding (large) x. By the
Bateman-Horn conjecture, the function pi^k_{2r}(x) should be asymptotic to
(2/k)C^k_{2r}li_2(x), with certain specific constants C^k_{2r}. Heuristic
arguments lead to the conjecture that these constants have mean value one, just
like the Hardy-Littlewood constants C_{2r} for prime pairs (p,p+2r). The
conjecture is supported by extensive numerical work.
