A063972 - OEIS

%I #10 Sep 22 2019 08:03:50

%S 1,0,2,3,4,5,0,7,8,9,0,6,0,13,0,0,16,10,0,12,0,0,0,14,0,25,0,27,0,18,

%T 0,21,32,0,0,22,0,37,0,28,0,26,0,43,0,0,0,33,0,36,0,0,0,34,0,39,0,0,0,

%U 38,0,61,0,0,64,0,0,48,0,52,0,30,0,73,0,0,0,50,0,57,0,81,0,54,0,0,0,0,0

%N a(n) is the smallest 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 Amiram Eldar, <a href="/A063972/b063972.txt">Table of n, a(n) for n = 1..10000</a>

%e a(12)=6 because the unitary divisors of 6 are 1,2,3,6; they sum to 12.

%t usigma[1] = 1; usigma[n_] := Times @@ (1 + Power @@@ FactorInteger[n]); m = 100; v = Table[0, {m}];  Do[u = usigma[n]; If[u <= m && v[[u]] == 0, v[[u]] = n], {n, 1, m}]; v (* _Amiram Eldar_, Sep 22 2019 *)

%Y Cf. A034444, A034448, A051444, A057637, A063973.

%K nonn

%O 1,3

%A _Labos Elemer_, Sep 05 2001

%E Edited by _Don Reble_, May 14 2006