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%I #13 Aug 26 2017 04:23:58
%S 4,20,115,633,4234,29949,225630,1762579,14176573,116718282,979244657,
%T 8343503219
%N Number of Chen primes up to 10^n.
%C Chen primes are primes p such that p + 2 is either prime or semiprime.
%H Jing Run Chen, <a href="https://doi.org/10.1142/9789814542487_0019">On the representation of a larger even integer as the sum of a prime and the product of at most two primes</a>, Sci. Sinica 16 (1973), pp. 157-176.
%o (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
%Y Cf. A109611.
%K nonn,hard,more
%O 1,1
%A _Charles R Greathouse IV_, Aug 25 2017
%E a(10)-a(12) from _Giovanni Resta_, Aug 26 2017
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