This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A291531 #13 Aug 26 2017 04:23:58 %S A291531 4,20,115,633,4234,29949,225630,1762579,14176573,116718282,979244657, %T A291531 8343503219 %N A291531 Number of Chen primes up to 10^n. %C A291531 Chen primes are primes p such that p + 2 is either prime or semiprime. %H A291531 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 A291531 (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 A291531 Cf. A109611. %K A291531 nonn,hard,more %O A291531 1,1 %A A291531 _Charles R Greathouse IV_, Aug 25 2017 %E A291531 a(10)-a(12) from _Giovanni Resta_, Aug 26 2017