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A193071
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Odd numbers N for which sigma(N^2) is not prime.
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2
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1, 7, 9, 11, 13, 15, 19, 21, 23, 25, 29, 31, 33, 35, 37, 39, 43, 45, 47, 51, 53, 55, 57, 61, 63, 65, 67, 69, 73, 75, 77, 79, 81, 83, 85, 87, 91, 93, 95, 97, 99, 103, 105, 107, 109, 111, 113, 115, 117, 119, 121, 123, 127, 129, 133, 135, 137, 139, 141, 143, 145, 147
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OFFSET
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1,2
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COMMENTS
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The function sigma(n) (=A000203(n)) takes odd values when n is a square or twice a square. Thus, odd numbers n for which sigma(n) is prime (cf. A023194) must be odd squares. This sequence gives the odd numbers whose square yields a composite sum of divisors (or 1).
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LINKS
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FORMULA
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MATHEMATICA
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Select[Range[1, 151, 2], !PrimeQ[DivisorSigma[1, #^2]]&] (* Harvey P. Dale, Dec 13 2012 *)
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PROG
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(PARI) forstep(N=1, 1e7, 2, !isprime(sigma(N^2)) && print1(N", "))
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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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