A329881 Nonunitary doubly superperfect numbers: numbers k such that nusigma(nusigma(k)) = 2*k, where nusigma(k) = sigma(k) - usigma(k) is the sum of nonunitary divisors of k (A048146).

4032, 13104, 58032, 69648, 237744, 278592, 365652, 1114368, 15333552, 71319552, 245364912, 981465264, 1141112832, 4564451328, 873139150710, 4020089387184

Offset

1,1

Comments

Analogous to superperfect numbers (A019279) as nonunitary doubly perfect numbers (A064592) is analogous to perfect numbers (A000396).

If n = 2^k*3*1451 and nusigma(n) = 2^5*3*11^2*p, with p > 11 prime, then n is a term. This happens for k = 4, 6, 8, 14, 18, 20, 32, 62, 90, 108, 128, 522, 608, ... . Similarly, if p=2^k-1 is prime (A000043), then 2^4*3^2*13*p is a term for k > 2. - Giovanni Resta, Nov 23 2019

a(17) > 6*10^12. - Giovanni Resta, Nov 24 2019

Links

Table of n, a(n) for n=1..16.

Mathematica

usigma[1] = 1; usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); nusigma[n_] := DivisorSigma[1, n] - usigma[n]; Select[Range[3*10^5], nusigma[nusigma[#]] == 2*# &]

Crossrefs

Cf. A000396, A019279, A034448, A038843, A048146, A064592, A328120, A329884.

Sequence in context: A258883 A352752 A249107 * A234838 A234834 A212601

Adjacent sequences: A329878 A329879 A329880 * A329882 A329883 A329884

Keyword

nonn,more

Author

Amiram Eldar, Nov 23 2019

Extensions

a(15)-a(16) from Giovanni Resta, Nov 24 2019

Status

approved