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 A255490 #20 Aug 18 2021 06:56:27
%S A255490 1,5,7,19,25,77,103,307,409,1229,1639,4915,6553,19661,26215,78643,
%T A255490 104857,314573,419431,1258291,1677721,5033165,6710887,20132659,
%U A255490 26843545,80530637,107374183,322122547,429496729,1288490189,1717986919,5153960755
%N A255490 The subsequence A247649(2^n-1).
%C A255490 Note that A247649 is not the Run Length Transform of this sequence - compare A253085.
%H A255490 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,3,0,4)
%F A255490 G.f.: (1+5*x+4*x^2+4*x^3)/(1-3*x^2-4*x^4).
%t A255490 LinearRecurrence[{0,3,0,4}, {1,5,7,19}, 40] (* _Georg Fischer_, Aug 18 2021 *)
%Y A255490 Cf. A247649, A253085.
%K A255490 nonn,easy
%O A255490 0,2
%A A255490 _N. J. A. Sloane_, Mar 03 2015