A055033 - OEIS

%I #11 Jul 24 2024 18:11:21

%S 1,4,5,6,12,20,9,10,18,30,20,30,24,36,36,18,30,72,30,72,33,50,36,50,

%T 42,96,40,54,72,90,33,48,68,84,68,78,60,120,72,84,96,132,60,120,120,

%U 90,68,90,78,168,90,144,84,160,90,90,102,180,120,216,96

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

%H Nathaniel Johnston, <a href="/A055033/b055033.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A034448(A034448(n)). - _Amiram Eldar_, Jul 24 2024

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

%o (PARI) usigma(n) = {my(f = factor(n)); prod(i = 1, #f~, 1 + f[i, 1]^f[i, 2]);}

%o a(n) = usigma(usigma(n)); \\ _Amiram Eldar_, Jul 24 2024

%Y Cf. A034448, A051027, A061765, A064012.

%K nonn,easy

%O 1,2

%A _N. J. A. Sloane_, Oct 30 2001