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Search: id:A061367
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| A061367 |
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Composite n such that sigma(n)-phi(n) divides sigma(n)+phi(n). |
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1
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| 15, 35, 95, 119, 143, 209, 287, 319, 323, 357, 377, 527, 559, 779, 899, 923, 989, 1007, 1045, 1189, 1199, 1343, 1349, 1763, 1919, 2159, 2261, 2507, 2639, 2759, 2911, 3239, 3339, 3553, 3599, 3827, 4031, 4147, 4607, 5049, 5183, 5207, 5249, 5459, 5543, 6439
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OFFSET
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1,1
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COMMENT
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Primes trivially satisfy the defining condition.
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FORMULA
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It seems that a(n) is asymptotic to c*n^2, 2<c<2.5 and that a(n)>2*n^2 - Benoit Cloitre (benoit7848c(AT)orange.fr), Sep 17 2002
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EXAMPLE
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sigma(15)-phi(15) = 24-8 = 16 divides sigma(15)-phi(15)=24+8 = 32, so 15 is a term of the sequence.
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MATHEMATICA
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f[n_] := Module[{a = DivisorSigma[1, n], b = EulerPhi[n]}, Mod[(a + b), (a - b)] == 0]; Select[Range[2, 10^4], (f[ # ] && ! PrimeQ[ # ]) &]
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CROSSREFS
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Sequence in context: A098271 A082663 A109068 this_sequence A070161 A074480 A037074
Adjacent sequences: A061364 A061365 A061366 this_sequence A061368 A061369 A061370
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KEYWORD
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nonn
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AUTHOR
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Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Feb 13 2002
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