The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #12 Nov 18 2024 05:25:06
%S 1512,1872,2352,3192,3780,4104,4560,4752,5880,6120,8160,8424,8820,
%T 11424,13056,15264,16464,16704,17160,17360,17760,18648,19680,19800,
%U 20880,22752,23616,24552,24864,27432,30336,30492,31200,32448,35328
%N Numbers k such that sigma (x) = k has exactly 12 solutions.
%H Amiram Eldar, <a href="/A060676/b060676.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 1512 = sigma(480) = sigma(636) = sigma(736) = sigma(748) = sigma(830) = sigma(902) = sigma(1006) = sigma(1105) = sigma(1255) = sigma(1391) = sigma(1411) = sigma(1511).
%t a = Table[ 0, {50000} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 50001, a[ [ s ] ]++ ], {n, 1, 50000} ]; Select[ Range[ 50000 ], a[ [ # ] ] == 12 & ]
%t Take[Sort[Transpose[Select[Tally[DivisorSigma[1,Range[100000]]],#[[2]] == 12&]][[1]]],50] (* _Harvey P. Dale_, Jan 18 2013 *)
%o (PARI) is(k) = invsigmaNum(k) == 12 \\ _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), A060678 (11), this sequence (12).
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Apr 18 2001