A119663 Triangular numbers with at most two distinct prime factors.

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 91, 136, 153, 171, 253, 325, 351, 496, 703, 1081, 1225, 1431, 1711, 1891, 2701, 3321, 3403, 4753, 5671, 7381, 8128, 12403, 13203, 13861, 15931, 18721, 25651, 29161, 29403, 31375, 32896, 34453, 38503, 49141

Offset

1,2

Comments

1 and 3 are the only terms with less than two prime factors.

Links

Klaus Brockhaus, Table of n, a(n) for n=1..1000

Example

a(6) = 3 * 7, a(7) = 2^2 * 7, a(8) = 2^2 * 3^2.

Mathematica

Select[Accumulate[Range[0, 320]] , PrimeNu[#]<3&] (* James C. McMahon, Sep 17 2024 *)

Prog

(PARI) for(n=1, 320, k=binomial(n+1, 2); if(omega(k)<=2, print1(k, ", "))) \\ Klaus Brockhaus, Jul 30 2006

Crossrefs

Cf. A000217, A068443, A005384.

Sequence in context: A069696 A025724 A025746 * A025715 A165145 A046489

Adjacent sequences: A119660 A119661 A119662 * A119664 A119665 A119666

Keyword

easy,nonn

Author

Greg Huber, Jul 28 2006

Extensions

More terms from Klaus Brockhaus, Jul 30 2006 and May 21 2008

Status

approved