%I #10 Apr 03 2023 10:36:09
%S 4,80,2753,112649,5536978,311350561,19222612234
%N Numbers of 2n-digit primes which are concatenations of n-digit primes (no leading zeros allowed).
%H C. K. Caldwell, <a href="https://t5k.org/curios/page.php?short=80">Prime Curios! 80</a>
%e n=1 gives these four primes: 23, 37, 53 and 73.
%o (PARI) for(n=1,5,pdn=vector(10^n-10^(n-1));i=1;for(k=10^(n-1)+1,10^n-3,if(isprime(k),pdn[i]=k;i++));s=0;for(l=1,i-1,for(m=1,i-1,if(isprime(pdn[l]*(10^n)+pdn[m]),s++)));print1(s",")) - Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008
%Y Cf. A000040.
%K nonn,base,more
%O 1,1
%A _Patrick De Geest_, Sep 15 2000
%E a(5) from Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008
%E a(6)-a(7) from _Donovan Johnson_, Dec 19 2010