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A077438
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Numbers k such that Sum_{d|k} mu(d) mu(n/d)^2 = -1.
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2
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4, 9, 25, 49, 121, 169, 289, 361, 529, 841, 900, 961, 1369, 1681, 1764, 1849, 2209, 2809, 3481, 3721, 4356, 4489, 4900, 5041, 5329, 6084, 6241, 6889, 7921, 9409, 10201, 10404, 10609, 11025, 11449, 11881, 12100, 12769, 12996, 16129, 16900
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OFFSET
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1,1
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COMMENTS
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Number of terms < 10^k: 2, 4, 12, 30, 98, 303, 957, ..., . (End)
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LINKS
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FORMULA
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Sum_{k>=1} 1/a(k) = 9/(2*Pi^2) = A088245.
Sum_{k>=1} 1/a(k)^2 = 15/(2*Pi^4). (End)
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MATHEMATICA
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fQ[n_] := Block[{d = Divisors@ n}, Plus @@ (MoebiusMu[#] MoebiusMu[n/#]^2 & /@ d) == -1]; Select[Range@17000, fQ] (* Robert G. Wilson v, Dec 28 2016 *)
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PROG
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(PARI) isok(n) = sumdiv(n, d, moebius(d)*moebius(n/d)^2) == -1; \\ Michel Marcus, Nov 08 2013
(PARI) is(n)=if(!issquare(n, &n), return(0)); my(f=factor(n)[, 2]); #f%2 && vecmax(f)==1 \\ Charles R Greathouse IV, Oct 16 2015
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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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