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 A051027 a(n) = sigma(sigma(n)) = sum of the divisors of the sum of the divisors of n. 67
 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 A073435 A324854 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 September 22 05:47 EDT 2019. Contains 327287 sequences. (Running on oeis4.)