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A057637
a(n) is the largest number k such that sigma(k) = n, where sigma is the sum of divisors function A000203, or 0 if no such k exists.
10
1, 0, 2, 3, 0, 5, 4, 7, 0, 0, 0, 11, 9, 13, 8, 0, 0, 17, 0, 19, 0, 0, 0, 23, 0, 0, 0, 12, 0, 29, 25, 31, 0, 0, 0, 22, 0, 37, 18, 27, 0, 41, 0, 43, 0, 0, 0, 47, 0, 0, 0, 0, 0, 53, 0, 39, 49, 0, 0, 59, 0, 61, 32, 0, 0, 0, 0, 67, 0, 0, 0, 71, 0, 73, 0, 0, 0, 45, 0, 79, 0, 0, 0, 83, 0, 0, 0, 0, 0, 89
OFFSET
1,3
COMMENTS
Right border of A299762. - Omar E. Pol, Mar 14 2018
LINKS
EXAMPLE
11 is the largest k such that sigma(k) = 12, so a(12) = 11.
MATHEMATICA
a[n_] := Module[{k = n}, While[k > 0 && DivisorSigma[1, k] != n, k--]; k]; Array[a, 90] (* Amiram Eldar, Jan 05 2020 *)
PROG
(PARI) A057637(n)=if(n=A085790_row(n), n[#n]) \\ M. F. Hasler, Sep 21 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, Oct 10 2000
STATUS
approved