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 A256438 Numbers m such that sigma(sigma(m-1)) = 2*(m-1). 7
 3, 5, 17, 65, 4097, 65537, 262145, 1073741825, 1152921504606846977, 309485009821345068724781057, 81129638414606681695789005144065, 85070591730234615865843651857942052865 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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. LINKS FORMULA a(n) = A019279(n) + 1. - Michel Marcus, Feb 09 2020 EXAMPLE 17 is in the sequence because sigma(sigma(17-1)) = 32 = 2*(17-1). MAPLE 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 MATHEMATICA Select[Range@ 1000000, DivisorSigma[1, DivisorSigma[1, # - 1]] == 2 (# - 1) &] (* Michael De Vlieger, Mar 29 2015 *) PROG (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 CROSSREFS Cf. A000203, A051027, A019279, A249759. Sequence in context: A281623 A278741 A265425 * A251737 A125957 A259908 Adjacent sequences:  A256435 A256436 A256437 * A256439 A256440 A256441 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Mar 29 2015 STATUS approved

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Last modified April 14 07:59 EDT 2021. Contains 342946 sequences. (Running on oeis4.)