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 A380969 #18 Feb 18 2025 17:39:26
%S A380969 0,1,3,13,47,307,2163,17557,191807,1413443,16485763,169053487
%N A380969 a(n) is the smallest k such that tau(k^2 + 1) is equal to 2^n, where tau = A000005 and a(n) = -1 if no such k exists.
%F A380969 a(n) = A353008(2^(n-1)) for n > 0.
%t A380969 a[n_]:=Module[{k=0}, While[DivisorSigma[0, k^2+1]!=2^n, k++]; k]; Array[a, 9,0] (* _Stefano Spezia_, Feb 10 2025 *)
%o A380969 (PARI) a(n) = my(k=0); while (numdiv(k^2+1) != 2^n, k++); k; \\ _Michel Marcus_, Feb 09 2025
%Y A380969 Cf. A000005, A164511, A180278, A353008, A380798.
%K A380969 nonn,hard,more
%O A380969 0,3
%A A380969 _Juri-Stepan Gerasimov_, Feb 09 2025
%E A380969 a(10)-a(11) from _Stefano Spezia_, Feb 12 2025