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A081339
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Numbers n such that sigma(n^2) modulo 4 = 1.
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1
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1, 3, 7, 9, 10, 11, 19, 20, 21, 23, 25, 26, 27, 30, 31, 33, 34, 40, 43, 47, 49, 52, 57, 58, 59, 60, 63, 65, 67, 68, 69, 70, 71, 74, 75, 77, 78, 79, 80, 81, 82, 83, 85, 90, 93, 99, 102, 103, 104, 106, 107, 110, 116, 120, 121, 122, 127, 129, 131, 133, 136, 139, 140, 141, 145
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OFFSET
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1,2
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COMMENTS
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Numbers n such that the sum of exponents of primes == 1 (mod 4) in the prime factorization of n is not congruent to n mod 2. - Robert Israel, Jan 22 2017
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LINKS
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MAPLE
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filter:= proc(n) local F, t;
F:= select(t -> t[1] mod 4 = 1, ifactors(n)[2]);
(add(t[2], t=F) - n) mod 2 = 1;
end proc:
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MATHEMATICA
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Select[Range[150], Mod[DivisorSigma[1, #^2], 4]==1&] (* Harvey P. Dale, Apr 07 2012 *)
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PROG
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(PARI) isok(n) = (sigma(n^2) % 4) == 1; \\ Michel Marcus, Jan 22 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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