A237352 a(1)=1; for n>1, a(n) is the smallest positive integer for which sigma(a(n)) is a proper multiple of sigma(a(n-1)).

1, 2, 5, 6, 14, 30, 66, 138, 210, 462, 930, 1974, 3810, 6510, 14322, 26670, 58674, 118110, 245730, 498666, 826770, 1720110, 3784242, 7617630, 15728610, 27524910, 53323410, 110100270, 218453970, 480598734, 853272210, 1877198862, 3413108370, 6772073070

Offset: 1

Author

Michel Marcus, Apr 22 2014

Keywords

• nonn

Comments

Subsequence of A090552.

The first term of A090552 that is not here is 11 since sigma(11) equals sigma(a(4)), so it is not a proper multiple.

Links

• Hiroaki Yamanouchi, Table of n, a(n) for n = 1..50

Crossrefs

Cf. A090552.

Programs

PARI

lista(nn) = {n = 1; precs = sigma(n); print1(n, ", "); for (n=2, nn, if ((((ns=sigma(n)) % precs) == 0) && (ns != precs), precs = ns; print1(n, ", ");););}

Extensions

a(33)-a(34) from Hiroaki Yamanouchi, Sep 30 2014