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A119663 Triangular numbers with at most two distinct prime factors. 3
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 (list; graph; refs; listen; history; text; internal format)
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.

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

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Last modified November 19 12:38 EST 2018. Contains 317351 sequences. (Running on oeis4.)