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 A133026 #9 Jul 09 2025 15:59:08
%S A133026 3,517,257655374294967297
%N A133026 Concatenation of next n Fermat numbers A000215.
%C A133026 a(4) has 292 digits and is too large to include.
%H A133026 Amiram Eldar, <a href="/A133026/b133026.txt">Table of n, a(n) for n = 1..4</a>
%t A133026 a[n_] := FromDigits[Flatten[IntegerDigits[Table[2^(2^(k - 1)) + 1, {k, n*(n - 1)/2 + 1, n*(n + 1)/2}]]]]; Array[a, 3] (* _Amiram Eldar_, Jul 09 2025 *)
%Y A133026 Cf. A000215, A053067, A133013.
%K A133026 base,easy,nonn
%O A133026 1,1
%A A133026 _Omar E. Pol_, Nov 09 2007