A175587 a(1) = 4; a(n) = smallest semiprime such that a(n) - a(n-1) is prime.

4, 6, 9, 14, 21, 26, 33, 35, 38, 49, 51, 58, 65, 82, 85, 87, 94, 111, 118, 121, 123, 134, 141, 143, 145, 158, 161, 166, 169, 206, 209, 214, 217, 219, 221, 226, 237, 254, 259, 262, 265, 267, 274, 287, 289, 291, 298, 301, 303, 305, 334, 339, 341, 346, 365, 382

Offset: 1

Author

Zak Seidov, Jul 17 2010

Keywords

• nonn

Comments

First differences: 2, 3, 5, 7, 5, 7, 2, 3, 11, 2, 7, 7, 17, 3, 2, 7, 17, 7, 3, 2, 11, 7, 2, 2, 13, 3, 5, 3, 37, 3, 5, 3, 2, 2, 5, 11, 17, 5, 3, 3, 2, 7, 13, 2, 2, 7, 3, 2, 2 (all primes).

Links

• Zak Seidov, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A001358 (semiprimes).

Programs

Mathematica

NestList[(p = 2; While[2 != PrimeOmega[q = # + p], p = NextPrime[p]]; q) &, 4, 50]

PARI

nxt(n)=forprime(p=2,, if(bigomega(n+p)==2, return(n+p))) \\ Charles R Greathouse IV, Jan 25 2017