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A063973
a(n) is the largest m such that usigma(m) = n (or 0 if no such m).
2
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
1,3
COMMENTS
usigma(m) is the sum of unitary divisors of m, A034448.
LINKS
EXAMPLE
a(12) = 11 because the unitary divisors of 11 are 1 and 11, and their sum is 12.
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 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Sep 05 2001
EXTENSIONS
Corrected by Don Reble, May 14 2006
STATUS
approved