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URL: https://oeis.org/A116005

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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
OFFSET
1,2
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 * A378740 A297435 A281058
KEYWORD
nonn
AUTHOR
Giovanni Resta, Feb 13 2006
EXTENSIONS
a(5) from Donovan Johnson, Mar 09 2014
STATUS
approved