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A215015 Number of sphenic twins up to 10^n. 1
0, 0, 11, 337, 4206, 43330, 417479, 3917508, 36358375, 336046778 (list; graph; refs; listen; history; text; internal format)
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..10.

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

AUTHOR

Martin Renner, Aug 06 2012

EXTENSIONS

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

STATUS

approved

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Last modified May 9 00:09 EDT 2021. Contains 343685 sequences. (Running on oeis4.)