1,5

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.

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

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

Cf. A243158.

Sequence in context: A280639 A319037 A067565 * A307717 A226782 A010635

Adjacent sequences: A243151 A243152 A243153 * A243155 A243156 A243157

nonn

Vladimir Shevelev, May 31 2014

a(25) corrected and more terms from Colin Barker, May 31 2014

approved