

A291531


Number of Chen primes up to 10^n.


0



4, 20, 115, 633, 4234, 29949, 225630, 1762579, 14176573, 116718282, 979244657, 8343503219
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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. 157176.


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*q2), s++))); forprime(q=5, N, if(qp==2, s++); p=q); s


CROSSREFS

Cf. A109611.
Sequence in context: A128327 A320615 A316298 * A171802 A100034 A192924
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



