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 A135561 #12 Jul 06 2022 19:40:04
%S A135561 3,7,1,15,1,3,1,31,1,3,1,7,1,3,1,63,1,3,1,7,1,3,1,15,1,3,1,7,1,3,1,
%T A135561 127,1,3,1,7,1,3,1,15,1,3,1,7,1,3,1,31,1,3,1,7,1,3,1,15,1,3,1,7,1,3,1,
%U A135561 255,1,3,1,7,1,3,1,15,1,3,1,7,1,3,1,31,1,3,1,7,1,3,1,15,1,3,1,7,1,3,1,63,1,3
%N A135561 a(n) = 2^A135560(n) - 1.
%H A135561 R. Zumkeller, <a href="/A135561/b135561.txt">Table of n, a(n) for n = 1..10000</a>
%F A135561 a(2^k) = 2^(k+2) - 1; a(2^k + 2^(k-1)) = 2^k - 1. - _Reinhard Zumkeller_, Mar 02 2008
%t A135561 f[n_] := 1 + IntegerExponent[n, 2] + Sum[(-1)^(n - k - 1)*Binomial[n - 1, k]* Sum[Binomial[k, 2^j - 1], {j, 0, k}], {k, 0, n - 1}]; Table[2^f[k] - 1, {k, 1, 20}] (* _G. C. Greubel_, Oct 17 2016 *)
%o A135561 (Python)
%o A135561 def A135561(n): return (1<<(m:=(~n & n-1)).bit_length()+int(m==n-1)+1)-1 # _Chai Wah Wu_, Jul 06 2022
%Y A135561 Cf. A007814, A036987, A091090, A135534, A135560.
%K A135561 nonn,easy
%O A135561 1,1
%A A135561 _N. J. A. Sloane_, Mar 01 2008
%E A135561 More terms from _Reinhard Zumkeller_, Mar 02 2008