A071868 Number of k (1 <= k <= n) such that k^2+1 is prime.

1, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 11, 11, 11, 11, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13, 13, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 15, 15, 15, 15, 15, 15, 15, 15, 16, 16, 16, 16

Offset

1,2

Links

Table of n, a(n) for n=1..77.

Formula

Hardy and Littlewood conjectured that : a(n) ~ c* sqrt(n)/Log(n) where c = prod(p prime, 1 - (-1)^((p-1)/2)/(p-1) ) = 1, 3727...

Mathematica

Accumulate[Table[If[PrimeQ[k^2+1], 1, 0], {k, 80}]] (* Harvey P. Dale, Jan 08 2020 *)

Prog

(PARI) for(n=1, 200, print1(sum(i=1, n, if(isprime(i^2+1)-1, 0, 1)), ", "))

Crossrefs

Cf. A005574, A002496.

Sequence in context: A276571 A066063 A123087 * A179390 A237819 A082447

Adjacent sequences: A071865 A071866 A071867 * A071869 A071870 A071871

Keyword

easy,nonn

Author

Benoit Cloitre, Jun 09 2002

Status

approved