A364840 Numbers k such that the product of the first k primes - 1 is a semiprime.

4, 7, 10, 11, 14, 17, 20, 21, 25, 26, 48, 51, 55, 85, 96, 127, 149, 150, 170

Offset: 1

Author

Sean A. Irvine, Oct 21 2023

Keywords

• more
• hard
• nonn

Crossrefs

Cf. A000720, A057588, A001358, A104876, A085725, A057704.

Programs

PARI

isok(k) = bigomega(-1 + prod(j=1, k, prime(j))) == 2;

Formula

a(n) = primepi(A104876(n)).