cp's OEIS Frontend

A063973 a(n) is the largest m such that usigma(m) = n (or 0 if no such m).

%I A063973 #14 Aug 27 2024 09:15:22
%S A063973 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 A063973 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 A063973 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 A063973 a(n) is the largest m such that usigma(m) = n (or 0 if no such m).
%C A063973 usigma(m) is the sum of unitary divisors of m, A034448.
%H A063973 Daniel Suteu, <a href="/A063973/b063973.txt">Table of n, a(n) for n = 1..10000</a>
%e A063973 a(12) = 11 because the unitary divisors of 11 are 1 and 11, and their sum is 12.
%t A063973 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 A063973 Cf. A034444, A034448, A051444, A057637, A063972.
%K A063973 nonn
%O A063973 1,3
%A A063973 _Labos Elemer_, Sep 05 2001
%E A063973 Corrected by _Don Reble_, May 14 2006