cp's OEIS Frontend

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.

%I A057637 #25 Aug 28 2023 11:48:03
%S A057637 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,
%T A057637 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,
%U A057637 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
%N 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.
%C A057637 Right border of A299762. - _Omar E. Pol_, Mar 14 2018
%H A057637 Amiram Eldar, <a href="/A057637/b057637.txt">Table of n, a(n) for n = 1..10000</a>
%e A057637 11 is the largest k such that sigma(k) = 12, so a(12) = 11.
%t A057637 a[n_] := Module[{k = n}, While[k > 0 && DivisorSigma[1, k] != n, k--]; k]; Array[a, 90] (* _Amiram Eldar_, Jan 05 2020 *)
%o A057637 (PARI) A057637(n)=if(n=A085790_row(n), n[#n]) \\ _M. F. Hasler_, Sep 21 2022
%Y A057637 Cf. A000203, A051444, A054973, A299762.
%K A057637 nonn
%O A057637 1,3
%A A057637 _Jud McCranie_, Oct 10 2000