A116048 sigma(n) times the n-th prime gives a triangular number.

5, 7, 15, 96

Offset

1,1

Comments

Probably finite.

No further terms up to n = 1 million. - Harvey P. Dale, Apr 15 2018

Links

Table of n, a(n) for n=1..4.

Example

prime(5) * sigma(5) = 11 * 6 = T(12).
prime(7) * sigma(7) = 17 * 8 = T(18).
prime(15) * sigma(15) = 47 * 24 = T(48).
prime(96) * sigma(96) = 503 * 252 = T(503).

Mathematica

Select[Range[200], OddQ[Sqrt[8(Prime[#]DivisorSigma[1, #])+1]]&] (* Harvey P. Dale, Apr 15 2018 *)

Prog

(PARI) isok(n) = ispolygonal(sigma(n)*prime(n), 3); \\ Michel Marcus, Jul 05 2014

Crossrefs

Sequence in context: A373305 A059613 A242503 * A120282 A132094 A082878

Adjacent sequences: A116045 A116046 A116047 * A116049 A116050 A116051

Keyword

nonn

Author

Giovanni Resta, Feb 13 2006

Status

approved