A215015 Number of sphenic twins up to 10^n.

0, 0, 11, 337, 4206, 43330, 417479, 3917508, 36358375, 336046778, 3105465308, 28739218426

Offset

1,3

Comments

The sphenic twin pairs {230, 231}, {285, 286}, ... are counted once at a time.

Links

Table of n, a(n) for n=1..12.

Lucas A. Brown, Python program.

Example

a(3) = 11 since there are 11 sphenic twins below 10^3 whose smaller members are 230, 285, 429, 434, 609, 645, 741, 805, 902, 969, 986.

Mathematica

sphQ[n_]:= FactorInteger[n][[;; , 2]] == {1, 1, 1}; c = 0; p = 10; q1 = 0; seq = {}; Do[q2 = sphQ[k]; If[q1 && q2, c++]; If[k == p, AppendTo[seq, c]; p*=10]; q1 = q2, {k, 2, 10^5}]; seq (* Amiram Eldar, Dec 26 2019 *)

Crossrefs

Cf. A007304, A215217, A066509, A215218, A215152.

Sequence in context: A086923 A206532 A106717 * A219072 A280669 A195505

Adjacent sequences: A215012 A215013 A215014 * A215016 A215017 A215018

Keyword

nonn,more,hard

Author

Martin Renner, Aug 06 2012

Extensions

a(8)-a(10) from Amiram Eldar, Dec 26 2019

a(11)-a(12) from Lucas A. Brown, Feb 12 2024

Status

approved