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%I #14 Jul 27 2016 10:03:46
%S 15,479,5,155257,3
%N Least k such that sigma(k*n) = sigma(k*n-1), or 0 if no such k exists.
%C See also A275337.
%C From _Michael De Vlieger_, Jul 27 2016: (Start)
%C Terms 6 through 60, with 0 signifying no such k <= 10^6: {0, 195, 99107, 23, 372601, 7257, 0, 105, 1439, 1, 312646, 158, 0, 31894, 0, 65, 4213, 9, 0, 119, 10559, 717, 59677, 5607, 0, 2086, 156323, 2419, 79, 39, 0, 16378, 15947, 35, 0, 362, 0, 12154, 544409, 97, 17236, 9362, 0, 15093, 0, 717, 261103, 4347, 0, 2681, 0, 38051, 3343, 1353, 0}.
%C n such that a(n) = 0 in above data: {6, 12, 18, 20, 24, 30, 36, 40, 42, 48, 50, 54, 56, 60}, i.e., multiples of 2 and 3, and 2 and 5. (End)
%e a(2) = 479 because sigma(479*2) = sigma(479*2-1).
%t Table[k = 1; While[DivisorSigma[1, k n] != DivisorSigma[1, k n - 1], k++]; k, {n, 5}] (* _Michael De Vlieger_, Jul 27 2016 *)
%o (PARI) a(n) = {my(k = 2); while (sigma(k*n) != sigma(k*n-1), k++); k; } \\ _Michel Marcus_, Jul 27 2016
%Y Cf. A002961, A275337.
%K nonn,more
%O 1,1
%A _Altug Alkan_, Jul 27 2016
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