1,1

C. K. Caldwell, Prime Curios! 80

n=1 gives these four primes 23, 37, 53 and 73.

(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

Cf. A000040.

Sequence in context: A055787 A132584 A012127 this_sequence A012824 A012057 A102063

Adjacent sequences: A057872 A057873 A057874 this_sequence A057876 A057877 A057878

nonn,base,more

Patrick De Geest (pdg(AT)worldofnumbers.com), Sep 15 2000.

a(5) from Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008