A116005 Numbers k such that k*sigma(k)*sigma(sigma(k)) is a triangular number.

1, 6, 67, 16447, 1073758207

Offset

1,2

Links

Table of n, a(n) for n=1..5.

Example

16447*sigma(16447)*sigma(sigma(16447)) = T(4210431).

Prog

(PARI) isok(n) = ispolygonal(n*sigma(n)*sigma(sigma(n)), 3); \\ Michel Marcus, Jan 27 2014

Crossrefs

Cf. A116004.

Sequence in context: A121958 A177555 A054746 * A297435 A281058 A152390

Adjacent sequences: A116002 A116003 A116004 * A116006 A116007 A116008

Keyword

nonn

Author

Giovanni Resta, Feb 13 2006

Extensions

a(5) from Donovan Johnson, Mar 09 2014

Status

approved