A318242 - OEIS

%I #19 Dec 10 2023 18:06:23

%S 1,2,8,15,24,42,240,648,168,480,321408,4320,57120,103680,1827840,

%T 23591520,898128000,374250240

%N a(n) is the least k such that A188999(A188999(k)) = n*k, where A188999 is the bi-unitary sigma function, or 0 if no such k exists.

%C It is also known that a(20) = 11975040.

%C Then for higher indices n, we have:

%C   a(19) <= 5235707393280;

%C   a(21) <= 49110437376000;

%C   a(22) <= 106780561395056640;

%C   a(24) <= 1099525819392000;

%C   a(25) <= 41252767395840;

%C   a(26) <= 202768780032000.

%H Tomohiro Yamada, <a href="https://arxiv.org/abs/1705.00189">2 and 9 are the only biunitary superperfect numbers</a>, arXiv:1705.00189 [math.NT], 2017. See Table 1.

%H Tomohiro Yamada, <a href="http://ac.inf.elte.hu/Vol_048_2018/247_48.pdf">2 and 9 are the only biunitary superperfect numbers</a>, Annales Univ. Sci. Budapest., Sec. Comp., Volume 48 (2018). See Table 1.

%Y Cf. A188999, A318175.

%Y Cf. A272930 (analog for sigma), A318272 (analog for infinitary sigma).

%K nonn,more

%O 1,2

%A _Michel Marcus_, Aug 22 2018