A243154 - OEIS

A243154

Smallest k>=0 such that prime(n)*prime(n+k)-2 is prime.

0, 0, 0, 0, 4, 0, 2, 0, 4, 0, 7, 0, 4, 0, 0, 4, 11, 0, 2, 0, 4, 7, 7, 0, 12, 4, 0, 0, 2, 9, 0, 0, 2, 0, 6, 2, 1, 4, 10, 0, 13, 4, 0, 4, 10, 4, 0, 0, 2, 8, 0, 0, 5, 6, 0, 30, 20, 16, 4, 11, 7, 0, 5, 13, 0, 11, 18, 0, 2, 18, 5, 0, 1, 4, 5, 10, 4, 7, 4, 5, 11

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OFFSET

1,5

COMMENTS

One of the possible illustrations of Chen's theorem: there are infinitely many primes p such that p+2 is semiprime.

For a dual sequence, see A243158.

LINKS

Table of n, a(n) for n=1..81.

MATHEMATICA

skp[n_]:=Module[{k=0, p=Prime[n]}, While[!PrimeQ[p Prime[n+k]-2], k++]; k]; Array[skp, 90] (* Harvey P. Dale, Aug 15 2021 *)

PROG

(PARI) vector(1000, n, k=0; while(!isprime(prime(n)*prime(n+k)-2), k++); k) \\ Colin Barker, May 31 2014