A291531 Number of Chen primes up to 10^n.

4, 20, 115, 633, 4234, 29949, 225630, 1762579, 14176573, 116718282, 979244657, 8343503219

Offset

1,1

Comments

Chen primes are primes p such that p + 2 is either prime or semiprime.

Links

Table of n, a(n) for n=1..12.

Jing Run Chen, On the representation of a larger even integer as the sum of a prime and the product of at most two primes, Sci. Sinica 16 (1973), pp. 157-176.

Prog

(PARI) a(n)=my(N=10^n+2, s, p=3); forprime(p=2, sqrtint(N), forprime(q=p, N\p, if(isprime(p*q-2), s++))); forprime(q=5, N, if(q-p==2, s++); p=q); s

Crossrefs

Cf. A109611.

Sequence in context: A128327 A320615 A316298 * A171802 A361245 A100034

Adjacent sequences: A291528 A291529 A291530 * A291532 A291533 A291534

Keyword

nonn,hard,more

Author

Charles R Greathouse IV, Aug 25 2017

Extensions

a(10)-a(12) from Giovanni Resta, Aug 26 2017

Status

approved