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 A168058 #12 Jan 30 2020 21:29:16
%S A168058 1,0,-2,-2,-4,-8,-18,-42,-102,-254,-646,-1670,-4376,-11596,-31022,
%T A168058 -83670,-227268,-621144,-1706934,-4713558,-13072764,-36398568,
%U A168058 -101704038,-285095118,-801526446,-2259520830,-6385455594,-18086805002
%N A168058 Expansion of x + sqrt(1-2x-3x^2).
%C A168058 a(n+2) = -2*A001006(n). Hankel transform is (-1)^n*A168057(n).
%C A168058 Essentially the same as A167022. - _R. J. Mathar_, Nov 18 2009
%H A168058 G. C. Greubel, <a href="/A168058/b168058.txt">Table of n, a(n) for n = 0..1000</a>
%F A168058 a(n) = 0^n - 2*Sum_{k=0..floor((n-2)/2)} C(n-2,2k)*A000108(k).
%F A168058 D-finite with recurrence: n*a(n) +(-2*n+3)*a(n-1) +3*(-n+3)*a(n-2)=0. - _R. J. Mathar_, Jan 23 2020
%e A168058 1 - 2*x^2 - 2*x^3 - 4*x^4 - 8*x^5 - 18*x^6 - 42*x^7 - 102*x^8 - 254*x^9 - ...
%t A168058 CoefficientList[Series[x + Sqrt[1 - 2 x - 3 x^2], {x, 0, 50}], x] (* _G. C. Greubel_, Jul 08 2016 *)
%Y A168058 Cf. A168055.
%K A168058 easy,sign
%O A168058 0,3
%A A168058 _Paul Barry_, Nov 17 2009