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 A253515 #17 Jan 15 2016 11:44:33
%S A253515 2,1,4,3,2,1,6,5,4,3,2,1,8,7,6,5,4,3,2,1,10,9,8,7,6,5,4,3,2,1,12,11,
%T A253515 10,9,8,7,6,5,4,3,2,1,14,13,12,11,10,9,8,7,6,5,4,3,2,1,16,15,14,13,12,
%U A253515 11,10,9,8,7,6,5,4,3,2,1,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1
%N A253515 Count down from 2*k to 1, then from 2*(k+1) to 1 and so on.
%F A253515 a(n) = 1 - n + floor(sqrt(n)+3/2)*floor(sqrt(n)+1/2).
%F A253515 a(n) = A130829(n) - A074294(n).
%t A253515 a253515[n_] := Block[{f}, f[x_] := 1 - x + Floor[Sqrt[x] + 3/2] * Floor[Sqrt[x] + 1/2]; Array[f, n]]; a253515[100] (* _Michael De Vlieger_, Jan 03 2015 *)
%t A253515 Flatten[Table[Range[2n,1,-1],{n,10}]] (* _Harvey P. Dale_, Jan 15 2016 *)
%Y A253515 Cf. A074294.
%K A253515 nonn
%O A253515 1,1
%A A253515 _Mikael Aaltonen_, Jan 03 2015