A356364 Number of primes p of the form k^2 + 1 less than 10^n such that p+2 and 2p+1 are also primes.

1, 1, 1, 1, 2, 3, 7, 10, 18, 43, 86, 185, 449, 1091, 2764, 6978, 17951, 47146, 125507, 337600, 916229, 2504458, 6898908

Offset

1,5

Links

Table of n, a(n) for n=1..23.

Example

For n = 5, a(5) = 2 since 5 and 25601 are the only two such primes less than or equal to 10^5.

Mathematica

seq[nmax_] := Module[{c = 0, pow = 10, s = {}, p}, Do[p = k^2 + 1; If[PrimeQ[p] && PrimeQ[p + 2] && PrimeQ[2*p + 1], c++]; If[p > pow, pow *= 10; AppendTo[s, c]], {k, 1, Floor[10^(nmax/2)] + 1}]; s]; seq[13] (* Amiram Eldar, Oct 16 2022 *)

Crossrefs

Cf. A006880, A007508, A083844, A092816, A347934.

Sequence in context: A100847 A271713 A355726 * A173132 A351704 A320675

Adjacent sequences: A356361 A356362 A356363 * A356365 A356366 A356367

Keyword

nonn,more

Author

Angad Singh, Oct 16 2022

Status

approved