Oh, that part I understand! It's because a(0)=true, and A019989 is a list of all n such that a(2n)=true. On Sun, Apr 14, 2019 at 1:56 PM Marc LeBrun <mlb at well.com> wrote: > For starters, why is it that A019989(0) is 2 and not 0? > > > -- > Seqfan Mailing list - http://list.seqfan.eu/ >