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

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, 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, 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

Offset: 1

Author

Labos Elemer, Sep 05 2001

Keywords

• nonn

Comments

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

Examples

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

Links

• Daniel Suteu, Table of n, a(n) for n = 1..10000

Crossrefs

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

Programs

Mathematica

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 *)

Extensions

Corrected by Don Reble, May 14 2006