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A116005 Numbers k such that k*sigma(k)*sigma(sigma(k)) is a triangular number. 1
1, 6, 67, 16447, 1073758207 (list; graph; refs; listen; history; text; internal format)
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

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Last modified December 1 10:54 EST 2021. Contains 349428 sequences. (Running on oeis4.)