A182265 - OEIS

%I #16 Apr 24 2012 03:14:51

%S 2,9,31,176,1057,7422,53709,407198,3198946,25773602,212205881,

%T 1777532673

%N Number of primes p < 10^n such that 4*p+1 is also prime.

%t f[n_] := Length[Select[Range[10^n], PrimeQ[#] && PrimeQ[4#+1]&]]; Table[f[n], {n,7}]

%o (PARI) a(n)=my(s=0); forprime(p=2,10^n,s+=isprime(4*p+1));s \\ _Charles R Greathouse IV_, Apr 23 2012

%Y Cf. A023212, A092816.

%K nonn

%O 1,1

%A _Enrique Pérez Herrero_, Apr 22 2012

%E a(10)-a(12) from _Charles R Greathouse IV_, Apr 23 2012