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%I #17 May 25 2023 17:51:57
%S 20,12,56,550,572,108,860,952,1232,6328,3708,40540,37072,79288,327260,
%T 357112,302000,527296,1764056,6506512,38559776,21893248,42257216,
%U 167771740,90798560,469761208,508198064,490304800,1353048560,2951488480,5067417200
%N a(n) is the smallest number k such that sigma(k) - 2k = 2^n.
%C For n > 31, a(n) > 1.724 * 10^10.
%e sigma(20) - 2*20 = 2^1, a(1) = 20.
%e sigma(108) - 2*108 = 64 = 2^6, a(6) = 108.
%t Table[k = 1; While[Log[2, DivisorSigma[1, k] - 2k] != n, k++]; k, {n, 30}]
%o (PARI) a(n) = my(k=1); while(sigma(k) - 2*k != 2^n, k++); k; \\ _Michel Marcus_, Sep 19 2017
%Y Cf. A000203, A033880, A088831, A088832, A088833, A141547, A175989, A275996.
%K nonn
%O 1,1
%A _XU Pingya_, Sep 19 2017
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