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A298563
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Numbers k such that k - 2 | sigma(k).
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0
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1, 3, 5, 6, 14, 44, 110, 152, 884, 2144, 8384, 18632, 116624, 8394752, 15370304, 73995392, 536920064, 2147581952, 34360131584
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OFFSET
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1,2
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COMMENTS
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Sequence includes every number of the form 2^(j-1)*(2^j+3) such that 2^j+3 is prime (i.e., j is a term in A057732); terms of this form are 5, 14, 44, 152, 2144, 8384, 8394752, 536920064, 2147581952, 34360131584, ... - Jon E. Schoenfield, Jan 22 2018
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LINKS
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EXAMPLE
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For k=44, sigma(k)/(k-2) = sigma(44)/(44-2) = 84/42 = 2, so 44 belongs to the sequence;
for k=110, sigma(k)/(k-2) = sigma(110)/(110-2) = 216/108 = 2, so 110 is also a term.
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MATHEMATICA
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Select[Range[10^6], Divisible[DivisorSigma[1, #], # - 2] &] (* Michael De Vlieger, Jan 21 2018 *)
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PROG
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(PARI) isok(k) = (k!=2) && !(sigma(k) % (k-2)); \\ Michel Marcus, Jan 22 2018
(Magma) [n: n in [3..10^7]| DivisorSigma(1, n) mod (n-2) eq 0]; // Vincenzo Librandi, Jan 22 2018
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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