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 A015756 a(n) is the least multiple of n, k*n say, such that phi(k) | sigma(k). 1
 1, 2, 3, 12, 15, 6, 14, 56, 270, 30, 264, 12, 78, 14, 15, 4064, 357, 270, 190, 140, 42, 264, 3956, 168, 27000, 78, 270, 56, 812, 30, 248, 4064, 264, 714, 35, 2376, 56536, 190, 78, 840, 2214, 42, 2580, 264, 270, 3956, 2109548, 12192, 56252, 27000, 357 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS MATHEMATICA a[n_] := Module[{k = n}, While[! Divisible[DivisorSigma[1, k], EulerPhi[k]], k += n]; k]; Array[a, 50] (* Amiram Eldar, Dec 10 2018 *) PROG (PARI) apply( A015756(n)=forstep(k=n, oo, n, sigma(k)%eulerphi(k)||return(k)), [1..50]) \\ M. F. Hasler, Dec 10 2018 CROSSREFS Sequence in context: A045878 A295398 A138321 * A163906 A302844 A180630 Adjacent sequences:  A015753 A015754 A015755 * A015757 A015758 A015759 KEYWORD nonn AUTHOR EXTENSIONS a(23) corrected by Sean A. Irvine, Dec 09 2018 Definition corrected by Sean A. Irvine and M. F. Hasler, Dec 10 2018 STATUS approved

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Last modified June 14 21:27 EDT 2021. Contains 345041 sequences. (Running on oeis4.)