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A291531 Number of Chen primes up to 10^n. 0
4, 20, 115, 633, 4234, 29949, 225630, 1762579, 14176573, 116718282, 979244657, 8343503219 (list; graph; refs; listen; history; text; internal format)
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 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

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Last modified July 4 15:25 EDT 2020. Contains 335448 sequences. (Running on oeis4.)