A364840 - OEIS

%I #9 Oct 21 2023 17:11:08

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

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

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

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

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

%K more,hard,nonn

%O 1,1

%A _Sean A. Irvine_, Oct 21 2023