A055033 a(n) = usigma(usigma(n)), where usigma(n) is the sum of unitary divisors of n (A034448).

1, 4, 5, 6, 12, 20, 9, 10, 18, 30, 20, 30, 24, 36, 36, 18, 30, 72, 30, 72, 33, 50, 36, 50, 42, 96, 40, 54, 72, 90, 33, 48, 68, 84, 68, 78, 60, 120, 72, 84, 96, 132, 60, 120, 120, 90, 68, 90, 78, 168, 90, 144, 84, 160, 90, 90, 102, 180, 120, 216, 96

Offset: 1

Author

N. J. A. Sloane, Oct 30 2001

Keywords

• nonn
• easy

Links

• Nathaniel Johnston, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A034448, A051027, A061765, A064012.

Programs

Mathematica

usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); usigma[1] = 1; a[n_] := usigma[usigma[n]]; Array[a, 100] (* Amiram Eldar, Jul 24 2024 *)

PARI

usigma(n) = {my(f = factor(n)); prod(i = 1, #f~, 1 + f[i, 1]^f[i, 2]);}
a(n) = usigma(usigma(n)); \\ Amiram Eldar, Jul 24 2024

Formula

a(n) = A034448(A034448(n)). - Amiram Eldar, Jul 24 2024