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A113930 Numbers n such that sigma(n) and phi(n) are both triangular numbers. 1
1, 2, 22, 3051, 3219, 3393, 5057, 8653, 75618, 95675, 100503, 102949, 104714, 287826, 438547, 522339, 537159, 688050, 2191200, 2317118, 2418548, 2507683, 2599128, 3212964, 4534573, 5367797, 6047913, 6302639, 7689149, 13758296, 14380145, 15342050, 16148979 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

phi(n)=A000010(n) is the Euler totient function, while sigma(n)=A000203(n) is the sum of divisors of n

LINKS

Donovan Johnson, Table of n, a(n) for n = 1..207 (terms < 10^12)

EXAMPLE

sigma(100503)=156520=T(559) and phi(100503)=61776=T(351).

PROG

(PARI) isok(n) = ispolygonal(sigma(n), 3) && ispolygonal(eulerphi(n), 3); \\ Michel Marcus, Jan 26 2014

CROSSREFS

Cf. A000010, A000203, A000217.

Subsequence of A045746.

Sequence in context: A182293 A222000 A261400 * A181235 A246852 A060601

Adjacent sequences:  A113927 A113928 A113929 * A113931 A113932 A113933

KEYWORD

nonn

AUTHOR

Giovanni Resta, Jan 30 2006

STATUS

approved

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Last modified June 21 16:42 EDT 2021. Contains 345365 sequences. (Running on oeis4.)