This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A292557 #25 Jan 11 2024 20:43:39
%S A292557 3,5,22,17,250,134,262,257,6556,4124,10330,8198,91036,19649,65542,
%T A292557 65537,1442716,524294,1363258,4194332,4411642,16442342,16866106,
%U A292557 22075325,156791188,536871032,2160104368,536870918,1074187546,2147483654,4295862586,19492545788
%N A292557 a(n) is the smallest number k such that 2k - sigma(k) = 2^n.
%C A292557 Primes of the form 2^n+1, i.e., Fermat primes (A019434) are terms of this sequence.
%C A292557 For n > 32, a(n) > 2 * 10^10.
%e A292557 sigma(20) - 2*20 = 2^1, a(1) = 20.
%e A292557 sigma(108) - 2*108 = 64 = 2^6, a(6) = 108.
%t A292557 Table[k = 1; While[Log[2, 2k - DivisorSigma[1, k]] != n, k++]; k, {n, 31}]
%o A292557 (PARI) a(n) = my(k=1); while(2*k - sigma(k) != 2^n, k++); k; \\ _Michel Marcus_, Sep 19 2017
%Y A292557 Cf. A019434, A033879, A125246, A125247, A125248, A191363, A275997.
%K A292557 nonn
%O A292557 1,1
%A A292557 _XU Pingya_, Sep 19 2017