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a(n) is the least multiple of n, k*n say, such that phi(k) | sigma(k).
1

%I #19 Jan 01 2019 06:57:51

%S 1,2,3,12,15,6,14,56,270,30,264,12,78,14,15,4064,357,270,190,140,42,

%T 264,3956,168,27000,78,270,56,812,30,248,4064,264,714,35,2376,56536,

%U 190,78,840,2214,42,2580,264,270,3956,2109548,12192,56252,27000,357

%N a(n) is the least multiple of n, k*n say, such that phi(k) | sigma(k).

%t a[n_] := Module[{k = n}, While[! Divisible[DivisorSigma[1, k], EulerPhi[k]], k += n]; k]; Array[a, 50] (* _Amiram Eldar_, Dec 10 2018 *)

%o (PARI) apply( A015756(n)=forstep(k=n,oo,n,sigma(k)%eulerphi(k)||return(k)), [1..50]) \\ _M. F. Hasler_, Dec 10 2018

%K nonn

%O 1,2

%A _Robert G. Wilson v_

%E a(23) corrected by _Sean A. Irvine_, Dec 09 2018

%E Definition corrected by _Sean A. Irvine_ and _M. F. Hasler_, Dec 10 2018