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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

Table of n, a(n) for n=1..51.

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

Robert G. Wilson v

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.)