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Search: id:A109421
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| A109421 |
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Numbers n such that tau(n)/bigomega(n) is an integer [tau(n)=number of divisors of n; bigomega(n)=number of prime divisors of n, counted with multiplicities]. |
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+0
2
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| 2, 3, 5, 6, 7, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, 26, 28, 29, 31, 33, 34, 35, 37, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 65, 67, 68, 69, 71, 73, 74, 75, 76, 77, 79, 80, 82, 83, 84, 85, 86, 87, 88, 89
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Integers greater than 1 and not in A109422.
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EXAMPLE
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12 is in the sequence because tau(12)=6 (1,2,3,4,6,12) and bigomega(12)=3 (2,2,3) and so tau(12)/bigomega(12)=2.
16 is not in the sequence because tau(16)=5 (1,2,4,8,16) and bigomega(16)=4 (2,2,2,2) and so tau(16)/bigomega(16)=5/4.
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MAPLE
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with(numtheory): a:=proc(n) if type(tau(n)/bigomega(n), integer)=true then n else fi end: seq(a(n), n=2..110);
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MATHEMATICA
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f[n_] := DivisorSigma[0, n]/Plus @@ Last /@ FactorInteger[n]; Select[ Range[2, 89], IntegerQ[ f[ # ]] &] (from Robert G. Wilson v (rgwv(AT)rgwv.com), Jun 29 2005)
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CROSSREFS
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Cf. A109422.
Sequence in context: A085971 A007916 A052485 this_sequence A065872 A028741 A119316
Adjacent sequences: A109418 A109419 A109420 this_sequence A109422 A109423 A109424
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KEYWORD
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nonn
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 28 2005
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