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A051027 a(n) = sigma(sigma(n)) = sum of the divisors of the sum of the divisors of n. 75
1, 4, 7, 8, 12, 28, 15, 24, 14, 39, 28, 56, 24, 60, 60, 32, 39, 56, 42, 96, 63, 91, 60, 168, 32, 96, 90, 120, 72, 195, 63, 104, 124, 120, 124, 112, 60, 168, 120, 234, 96, 252, 84, 224, 168, 195, 124, 224, 80, 128, 195, 171, 120, 360, 195, 360, 186, 234, 168, 480, 96 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If n is prime, a(n) = sigma(n+1) = A000203(n+1). - Wesley Ivan Hurt, Feb 14 2014

a(n) = 2*n iff n = 2^q with M_(q+1) = 2^(q+1) - 1 is a Mersenne prime, hence iff n = 2^q with q in A090748. - Bernard Schott, Aug 08 2019

LINKS

T. D. Noe, Table of n, a(n) for n=1..5000

FORMULA

a(n) = A000203(A000203(n)). - Zak Seidov, Aug 29 2012

EXAMPLE

a(2) = 4 because sigma(2)=1+2=3 and sigma(3)=1+3=4. - Zak Seidov, Aug 29 2012

MAPLE

with(numtheory): [seq(sigma(sigma(n)), n=1..100)];

MATHEMATICA

DivisorSigma[1, DivisorSigma[1, Range[100]]] (* Zak Seidov, Aug 29 2012 *)

PROG

(PARI) a(n)=sigma(sigma(n)); \\ Joerg Arndt, Feb 16 2014

CROSSREFS

Cf. A000203.

Sequence in context: A093494 A310934 A310935 * A291402 A328792 A073435

Adjacent sequences:  A051024 A051025 A051026 * A051028 A051029 A051030

KEYWORD

easy,nice,nonn,look

AUTHOR

Judson D. Neer (judson(AT)poboxes.com)

STATUS

approved

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Last modified April 1 10:33 EDT 2020. Contains 333159 sequences. (Running on oeis4.)