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
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