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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 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 A341919 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 12:21 EDT 2022. Contains 355075 sequences. (Running on oeis4.)