A175532 a(1) = 1; a(2) = 9; for n>2, a(n) = smallest number of the form 4k+1 and a(n) > a(n-1) such that the pairwise sums of elements are all semiprimes.

1, 9, 13, 25, 133, 193, 18673, 57313, 1032313, 4387273, 3450430573, 11717813053, 21691948933

Offset: 1

Author

Zak Seidov, Jun 12 2010

Keywords

• nonn
• more

Comments

Or, for i<>j, (a(i)+a(j))/2 are primes.

Starting with a(5)=133 on all terms === 13 mod 20.

Is this sequence finite?

Crossrefs

Cf. A115760.

Extensions

a(11)-a(13) from Jason Yuen, Oct 13 2024