OFFSET
1,2
COMMENTS
Additionally, there exist longer chains of 3 or 4 elements; e.g.,
- 8 (3), 15 (4), 24 (5), 60 (6);
- 9 (2), 10 (3), 18 (4), 30 (5);
- 512 (3), 1023 (4), 1536 (5), 4092 (6);
- 8925 (4), 14976 (5), 35700 (6);
- 219969739395000 (16), 899826278400000 (17), 3519515830320000 (18).
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..227 (terms < 10^12; first 185 terms from Tomohiro Yamada)
Tomohiro Yamada, 2 and 9 are the only biunitary superperfect numbers, arXiv:1705.00189 [math.NT], 2017. See Table 1.
Tomohiro Yamada, 2 and 9 are the only biunitary superperfect numbers, Annales Univ. Sci. Budapest., Sec. Comp., Volume 48 (2018). See Table 1.
Michel Marcus, Unexhaustive list of terms.
EXAMPLE
MATHEMATICA
bsigma[n_] := If[n==1, 1, Product[{p, e} = pe; If[OddQ[e], (p^(e+1)-1)/(p-1), ((p^(e+1)-1)/(p-1)-p^(e/2))], {pe, FactorInteger[n]}]];
Reap[For[m = 1, m < 20000, m++, If[Divisible[bsigma @ bsigma @ m, m], Sow[m]]]][[2, 1]] (* Jean-François Alcover, Sep 22 2018 *)
PROG
(PARI) a188999(n) = {f = factor(n); for (i=1, #f~, p = f[i, 1]; e = f[i, 2]; f[i, 1] = if (e % 2, (p^(e+1)-1)/(p-1), (p^(e+1)-1)/(p-1) -p^(e/2)); f[i, 2] = 1; ); factorback(f); }
isok(n) = frac(a188999(a188999(n))/n) == 0;
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Aug 20 2018
STATUS
approved