%I A057875
%S A057875 4,80,2753,112649,5536978
%N A057875 Numbers of 2n-digit primes which are concatenations of n-digit primes 
               (no leading zeros allowed).
%H A057875 C. K. Caldwell, <a href="http://primes.utm.edu/curios/page.php?short=80">
               Prime Curios! 80</a>
%e A057875 n=1 gives these four primes 23, 37, 53 and 73.
%o A057875 (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 A057875 Cf. A000040.
%Y A057875 Sequence in context: A055787 A132584 A012127 this_sequence A012824 A012057 
               A102063
%Y A057875 Adjacent sequences: A057872 A057873 A057874 this_sequence A057876 A057877 
               A057878
%K A057875 nonn,base,more
%O A057875 1,1
%A A057875 Patrick De Geest (pdg(AT)worldofnumbers.com), Sep 15 2000.
%E A057875 a(5) from Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008