A057875 Numbers of 2n-digit primes which are concatenations of n-digit primes (no leading zeros allowed).

4, 80, 2753, 112649, 5536978, 311350561, 19222612234

Offset

1,1

Links

Table of n, a(n) for n=1..7.

C. K. Caldwell, Prime Curios! 80

Example

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

Prog

(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

Crossrefs

Cf. A000040.

Sequence in context: A277074 A012127 A189791 * A187543 A268734 A012824

Adjacent sequences: A057872 A057873 A057874 * A057876 A057877 A057878

Keyword

nonn,base,more

Author

Patrick De Geest, Sep 15 2000

Extensions

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

a(6)-a(7) from Donovan Johnson, Dec 19 2010

Status

approved