A215152 Number of sphenic triples up to 10^n.

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

Offset: 1

Author

Martin Renner, Aug 06 2012

Keywords

• nonn
• more

Comments

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

Crossrefs

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

Programs

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

Extensions

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

a(11) from Amiram Eldar, Jan 15 2025