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 A051027 a(n) = sigma(sigma(n)) = sum of the divisors of the sum of the divisors of n. 76

%I

%S 1,4,7,8,12,28,15,24,14,39,28,56,24,60,60,32,39,56,42,96,63,91,60,168,

%T 32,96,90,120,72,195,63,104,124,120,124,112,60,168,120,234,96,252,84,

%U 224,168,195,124,224,80,128,195,171,120,360,195,360,186,234,168,480,96

%N a(n) = sigma(sigma(n)) = sum of the divisors of the sum of the divisors of n.

%C If n is prime, a(n) = sigma(n+1) = A000203(n+1). - _Wesley Ivan Hurt_, Feb 14 2014

%C 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

%H T. D. Noe, <a href="/A051027/b051027.txt">Table of n, a(n) for n=1..5000</a>

%F a(n) = A000203(A000203(n)). - _Zak Seidov_, Aug 29 2012

%e a(2) = 4 because sigma(2)=1+2=3 and sigma(3)=1+3=4. - _Zak Seidov_, Aug 29 2012

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

%t DivisorSigma[1,DivisorSigma[1,Range]] (* _Zak Seidov_, Aug 29 2012 *)

%o (PARI) a(n)=sigma(sigma(n)); \\ _Joerg Arndt_, Feb 16 2014

%Y Cf. A000203.

%K easy,nice,nonn,look

%O 1,2

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

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Last modified June 1 01:43 EDT 2020. Contains 334758 sequences. (Running on oeis4.)