

A108815


Indices of triangular numbers which are products of 3 primes.


6



7, 9, 11, 12, 14, 17, 18, 19, 21, 25, 28, 29, 30, 33, 34, 38, 41, 42, 43, 52, 57, 66, 67, 70, 78, 85, 86, 93, 94, 97, 101, 102, 109, 113, 118, 121, 122, 130, 133, 137, 138, 141, 142, 145, 148, 158, 163, 172, 173, 177, 181, 190, 201, 202, 205, 211, 213, 214, 217, 218
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OFFSET

1,1


COMMENTS

Indices of 3almost prime triangular numbers.


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Triangular Number.
Eric Weisstein's World of Mathematics, Almost Prime.


FORMULA

{a(n)} = {k such that A001222(A000217(k)) = 3}. {a(n)} = {k such that k*(k+1)/2 has exactly 3 prime factors, with multiplicity}. {a(n)} = {k such that A000217(k) is an element of A014612}.
n such that n*(n+1)/2 is an element of A014612. n such that A000217(n) is an element of A014612. n such that C(n+1, 2) is an element of A014612.
{ m : A069904(m) = 3 }.  Alois P. Heinz, Aug 05 2019


EXAMPLE

a(1) = 7 because T(7) = TriangularNumber(7) = 7*(7+1)/2 = 28 = 2^2 * 7 is a 3almost prime.
a(2) = 9 because T(9) = 9*(9+1)/2 = 45 = 3^2 * 5 is a 3almost prime.
a(3) = 11 because T(11) = 11*(11+1)/2 = 66 = 2 * 3 * 11.
a(31) = 101 because T(101) = 101*(101+1)/2 = 5151 = 3 * 17 * 101.
a(49) = 173 because T(173) = 173*(173+1)/2 = 15051 = 3 * 29 * 173.


MATHEMATICA

Select[Range[225], Plus @@ Last /@ FactorInteger[ #*(# + 1)/2] == 3 &] (* Ray Chandler, Jul 16 2005 *)


PROG

(PARI) issemi(n)=bigomega(n)==2
is(n)=if(isprime(n/gcd(n, 2)), issemi((n+1)/gcd(n+1, 2)), isprime((n+1)/gcd(n+1, 2)) && issemi(n/gcd(n, 2))) \\ Charles R Greathouse IV, Feb 05 2017


CROSSREFS

Cf. A000217, A001222, A014612, A069904.
Sequence in context: A283056 A162308 A191883 * A262536 A161992 A167377
Adjacent sequences: A108812 A108813 A108814 * A108816 A108817 A108818


KEYWORD

easy,nonn


AUTHOR

Jonathan Vos Post, Jul 10 2005


EXTENSIONS

Extended by Ray Chandler, Jul 16 2005
Edited by N. J. A. Sloane, May 07 2007


STATUS

approved



