A175588 a(1) = 4; a(n) = smallest semiprime > a(n-1) such that a(n) - a(n-1) is semiprime.

4, 10, 14, 35, 39, 49, 55, 65, 69, 91, 95, 121, 142, 146, 155, 159, 169, 178, 187, 201, 205, 209, 213, 217, 221, 235, 249, 253, 259, 265, 274, 278, 287, 291, 295, 299, 303, 309, 319, 323, 327, 341, 355, 361, 365, 371, 377, 381, 391, 395, 417, 427, 437, 446

Offset: 1

Author

Zak Seidov, Jul 17 2010

Keywords

• nonn

Comments

Subsequence of semiprimes with semiprime gaps.

Links

• Michael De Vlieger, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A175587 (subsequence of semiprimes with prime gaps), A001358 (semiprimes).

Programs

Mathematica

Nest[Append[#, Block[{k = #[[-1]] + 1}, While[! AllTrue[{k, k - #[[-1]]}, PrimeOmega[#] == 2 &], k++]; k]] &, {4}, 53] (* Michael De Vlieger, Dec 23 2019 *)

PARI

(nxt_A175588(t)=for(d=4,oo,bigomega(d)==2&&bigomega(t+d)==2&&return(t+d))); vector(99, i, t=nxt_A175588(if(i>1,t))) \\ M. F. Hasler, Dec 22 2019