

A090552


a(1)=1; for n>1, a(n) is the smallest positive integer for which sigma(a(n)) is a multiple of sigma(a(n1)).


4



1, 2, 5, 6, 11, 14, 15, 23, 30, 46, 51, 55, 66, 70, 94, 115, 119, 138, 154, 165, 210, 282, 310, 322, 345, 357, 382, 385, 462, 658, 682, 705, 759, 766, 805, 930, 966, 1146, 1155, 1270, 1426, 1491, 1551, 1581, 1645, 1705, 1771, 1915, 1974, 2046, 2170, 2298, 2415
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OFFSET

1,2


LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..61


EXAMPLE

After a(4)=6, with sigma(6)=12, the smallest number k such that sigma(k) is a multiple of 12 is 11, with sigma(11)=12. Thus a(5)=11.


MATHEMATICA

msa[n_]:=Module[{s1=DivisorSigma[1, n], n2=n+1}, While[Mod[DivisorSigma[1, n2], s1]!=0, n2++]; n2]; NestList[msa, 1, 60] (* Harvey P. Dale, Mar 20 2020 *)


PROG

(PARI) lista(nn) = {a = 1; print1(a, ", "); for (n = 1, nn, sa = sigma(a); a++; while (sigma(a) % sa, a++); print1(a, ", "); ); } \\ Michel Marcus, Oct 12 2013


CROSSREFS

Cf. A090553.
Sequence in context: A319242 A323398 A233865 * A024520 A340001 A015613
Adjacent sequences: A090549 A090550 A090551 * A090553 A090554 A090555


KEYWORD

nonn


AUTHOR

Amarnath Murthy, Dec 09 2003


EXTENSIONS

Corrected, extended and edited by John W. Layman, Aug 10 2004


STATUS

approved



