A210684 Number of primes p < 10^n such that both 2*p+1 and 4*p+1 are composite.

0, 7, 101, 864, 7365, 63331, 554839, 4931118, 44339730, 402709395, 3687732409, 34007530868

Offset: 1

Author

Enrique Pérez Herrero, May 09 2012

Keywords

• nonn

Crossrefs

Cf. A006880, A092816, A182265, A156660, A005384, A156874.

Programs

Mathematica

f[n_] := Length[Select[Range[10^n], PrimeQ[#] && !PrimeQ[2#+1] && !PrimeQ[4#+1]&]]; Table[f[n], {n,7}]

Formula

a(n) = A006880(n) - A092816(n) - A182265(n) + 1.

a(n) ~ 10^n / (n log 10). - Charles R Greathouse IV, May 11 2012