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A256438
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Numbers m such that sigma(sigma(m-1)) = 2*(m-1).
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7
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3, 5, 17, 65, 4097, 65537, 262145, 1073741825, 1152921504606846977, 309485009821345068724781057, 81129638414606681695789005144065, 85070591730234615865843651857942052865
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OFFSET
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1,1
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COMMENTS
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Numbers n such that A051027(n-1) = 2*(n-1).
Conjecture: numbers n of the form 2^k+1 such that sigma(2^k) = prime p.
Prime terms: 3, 5, 17, 65537, ...
Supersequence of A249759.
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LINKS
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Table of n, a(n) for n=1..12.
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FORMULA
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a(n) = A019279(n) + 1. - Michel Marcus, Feb 09 2020
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EXAMPLE
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17 is in the sequence because sigma(sigma(17-1)) = 32 = 2*(17-1).
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MAPLE
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with(numtheory): A256438:=n->`if`(sigma(sigma(n-1)) = 2*(n-1), n, NULL): seq(A256438(n), n=2..10^5); # Wesley Ivan Hurt, Mar 30 2015
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MATHEMATICA
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Select[Range@ 1000000, DivisorSigma[1, DivisorSigma[1, # - 1]] == 2 (# - 1) &] (* Michael De Vlieger, Mar 29 2015 *)
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PROG
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(MAGMA) [n: n in[2..10000000] | SumOfDivisors(SumOfDivisors(n-1)) eq 2*(n-1)]
(PARI) isok(m) = sigma(sigma(m-1)) == 2*(m-1); \\ Michel Marcus, Feb 09 2020
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CROSSREFS
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Cf. A000203, A051027, A019279, A249759.
Sequence in context: A281623 A278741 A265425 * A251737 A125957 A259908
Adjacent sequences: A256435 A256436 A256437 * A256439 A256440 A256441
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KEYWORD
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nonn,more
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AUTHOR
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Jaroslav Krizek, Mar 29 2015
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STATUS
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approved
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