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%I #11 Nov 18 2024 05:25:14
%S 576,1296,2976,3168,3648,3720,4788,4896,5544,6300,9000,9840,10656,
%T 11808,12528,13020,13320,14760,15456,16740,17920,18288,18576,19344,
%U 19840,20400,21280,22800,23296,24300,26712,26928,27552,27936,28392
%N Numbers k such that sigma (x) = k has exactly 11 solutions.
%H Amiram Eldar, <a href="/A060678/b060678.txt">Table of n, a(n) for n = 1..10000</a>
%H Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%e 576 = sigma(210) = sigma(282) = sigma(310) = sigma(322) = sigma(345) = sigma(357) = sigma(382) = sigma(385) = sigma(497) = sigma(517) = sigma(527).
%t a = Table[ 0, {30000} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 30001, a[ [ s ] ]++ ], {n, 1, 30000} ]; Select[ Range[ 30000 ], a[ [ # ] ] == 11 & ]
%o (PARI) is(k) = invsigmaNum(k) == 11 \\ _Amiram Eldar_, Nov 18 2024, using _Max Alekseyev_'s invphi.gp
%Y Cf. A000203.
%Y Number of solutions: A007369 (0), A007370 (1), A007371 (2), A007372 (3), A060660 (4), A060661 (5), A060662 (6), A060663 (7), A060664 (8), A060665 (9), A060666 (10), this sequence (11), A060676 (12).
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Apr 18 2001