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%I #34 Feb 15 2024 10:03:12
%S 0,0,11,337,4206,43330,417479,3917508,36358375,336046778,3105465308,
%T 28739218426
%N Number of sphenic twins up to 10^n.
%C The sphenic twin pairs {230, 231}, {285, 286}, ... are counted once at a time.
%H Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/A215015.py">Python program</a>.
%e 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.
%t 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 *)
%Y Cf. A007304, A215217, A066509, A215218, A215152.
%K nonn,more,hard
%O 1,3
%A _Martin Renner_, Aug 06 2012
%E a(8)-a(10) from _Amiram Eldar_, Dec 26 2019
%E a(11)-a(12) from _Lucas A. Brown_, Feb 12 2024
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