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 A189415 #22 Feb 16 2025 08:33:14 %S A189415 0,1,50,490,2618,9519,28432,70796,157912,321161,610482,1082570, %T A189415 1848362,3003015,4716792,7204604,10730528,15530189,22093410,30723078, %U A189415 42146178,56981411,75952240,99685104,129757248 %N A189415 Number of trapezoids on an n X n grid (or geoboard). %H A189415 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Trapezoid.html">Trapezoid</a>. %H A189415 Nathaniel Johnston, <a href="/A189415/a189415.c.txt">C program for computing terms</a>. %Y A189415 Cf. A181945, A186434, A189413, A189414, A189416, A189418. %K A189415 nonn %O A189415 1,3 %A A189415 _Martin Renner_, Apr 21 2011 %E A189415 a(6)-a(25) from _Nathaniel Johnston_, Apr 25 2011