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 A027121 #7 Oct 23 2019 20:34:42 %S A027121 1,4,13,40,120,352,1022,2948,8473,24304,69649,199544,571795,1639244, %T A027121 4702499,13500396,38790727,111555296,321100627,925086012,2667533646, %U A027121 7698716068,22238169212,64289860528,186010834243,538611262492 %N A027121 a(n) = A027113(n, 2n-3). %K A027121 nonn %O A027121 2,2 %A A027121 _Clark Kimberling_