A063973 - OEIS

%I #14 Aug 27 2024 09:15:22

%S 1,0,2,3,4,5,0,7,8,9,0,11,0,13,0,0,16,17,0,19,0,0,0,23,0,25,0,27,0,29,

%T 0,31,32,0,0,24,0,37,0,28,0,41,0,43,0,0,0,47,0,49,0,0,0,53,0,39,0,0,0,

%U 59,0,61,0,0,64,0,0,67,0,52,0,71,0,73,0,0,0,50,0,79,0,81,0,83,0,0,0,0

%N a(n) is the largest m such that usigma(m) = n (or 0 if no such m).

%C usigma(m) is the sum of unitary divisors of m, A034448.

%H Daniel Suteu, <a href="/A063973/b063973.txt">Table of n, a(n) for n = 1..10000</a>

%e a(12) = 11 because the unitary divisors of 11 are 1 and 11, and their sum is 12.

%t usigma[1] = 1; usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); a[n_] := Module[{k = n}, While[k > 0 && usigma[k] != n, k--]; k]; Array[a, 100]  (* _Amiram Eldar_, Aug 27 2024 *)

%Y Cf. A034444, A034448, A051444, A057637, A063972.

%K nonn

%O 1,3

%A _Labos Elemer_, Sep 05 2001

%E Corrected by _Don Reble_, May 14 2006