A291531 Number of Chen primes up to 10^n.

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

Offset: 1

Author

Charles R Greathouse IV, Aug 25 2017

Keywords

• nonn
• hard
• more

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.

Crossrefs

Cf. A109611.

Programs

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

Extensions

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