A066509 -id:A066509 - cp's OEIS frontend

A215152 Number of sphenic triples up to 10^n.

0, 0, 0, 21, 445, 5457, 55576, 527138, 4824694, 43484124, 389855718

Offset: 1

Martin Renner, Aug 06 2012

The sphenic triples {1309, 1310, 1311}, {1885, 1886, 1887}, ... are counted once at a time.

Cf. A007304, A215217, A066509, A215218, A215015.

issemiprime(n) = factor(n)[,2]~ == [1,1];
issphenic(n) = factor(n)[,2]~ == [1,1,1];
list(nmax) = {my(c = 0, pow = 10, lim = 10^nmax/2+1); forstep(k = 1, lim, 2, if(issemiprime(k) && issphenic(2*k-1) && issphenic(2*k+1), c++); if(2*k-1 > pow, print1(c, ", "); pow *= 10));} \\ Amiram Eldar, Jan 15 2025

a(8)-a(10) from Bert Dobbelaere, Jul 15 2023

a(11) from Amiram Eldar, Jan 15 2025

A165936 A sequence of triples of squarefree consecutive integers each composed of exactly three primes.

Original entry on oeis.org

1309, 1310, 1311, 1885, 1886, 1887, 2013, 2014, 2015, 2665, 2666, 2667, 3729, 3730, 3731, 5133, 5134, 5135, 6061, 6062, 6063, 6213, 6214, 6215, 6305, 6306, 6307, 6477, 6478, 6479, 6853, 6854, 6855, 6985, 6986, 6987, 7257, 7258, 7259, 7953, 7954, 7955, 8393

Offset: 1

Richard L. Peterson (rl_pete(AT)yahoo.com), Oct 01 2009

nonn

			1309 = 7 * 11 * 17, 1310 = 2 * 5 * 131, 1311 = 3 * 19 * 23.

a:=[]; f:=func; for k in [2..8500] do  if f(k) then a:=a cat [k, k+1, k+2]; end if;  end for; a; // Marius A. Burtea, Oct 05 2019

Select[Partition[Range[10000],3,1],AllTrue[#,SquareFreeQ]&&Union[ PrimeOmega[ #]] == {3}&]//Flatten (* Harvey P. Dale, Oct 02 2017 *)

a(3n+1) = A066509(n+1); a(3n+2) = 1 + a(3n+1); a(3n+3) = 1 + a(3n+2). - R. J. Mathar, Nov 27 2011

More terms from Harvey P. Dale, Oct 02 2017

cp's OEIS Frontend

A215152 Number of sphenic triples up to 10^n.

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A165936 A sequence of triples of squarefree consecutive integers each composed of exactly three primes.

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