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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
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.
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
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Mar 29 2015
STATUS
approved