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A087979
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a(n) = Min {x : sigma[x]=phi[nx], x is not a prime}, least nonprime solutions to sigma[x]=phi[nx].
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2
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1, 1, 15, 14, 14, 6, 6, 42, 30, 42, 168, 210, 210, 420, 840, 20790, 20790, 9240, 9240, 83160, 120120, 3984120, 5165160, 43825320, 26860680, 43825320, 1304863560, 569729160, 569729160, 16522145640, 18176198040
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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EXAMPLE
| n=4: a(4)=14, sigma[14]=24=phi[4.14]=phi[56],
n=5: a(5)=14, sigma[14]=24=phi[5.14]=phi[70].
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MATHEMATICA
| ds[x_, de_] := DivisorSigma[1, x]-EulerPhi[de*x] a[n_] := Block[{m=1, s=ds[m, n]}, While[(s !=0||PrimeQ[m])&&!Greater[m, 4000000], m++ ]; m]; Table[a[n], {n, 22}]
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CROSSREFS
| Cf. A074891, A000203, A000010, A088830.
Sequence in context: A023457 A004456 A055126 * A048294 A201345 A069785
Adjacent sequences: A087976 A087977 A087978 * A087980 A087981 A087982
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KEYWORD
| nonn
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AUTHOR
| Labos E. (labos(AT)ana.sote.hu), Sep 29 2003
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EXTENSIONS
| More terms from David Wasserman (wasserma(AT)spawar.navy.mil), Jun 20 2005
a(26)-a(31) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Feb 06 2010
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