A215152 Number of sphenic triples up to 10^n.

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

Offset

1,4

Comments

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

Links

Table of n, a(n) for n=1..11.

Prog

(PARI) 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

Crossrefs

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

Sequence in context: A285873 A189433 A041842 * A383702 A199252 A199197

Adjacent sequences: A215149 A215150 A215151 * A215153 A215154 A215155

Keyword

nonn,more

Author

Martin Renner, Aug 06 2012

Extensions

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

a(11) from Amiram Eldar, Jan 15 2025

Status

approved