A054139 T(2n,n), array T as in A054134.
1, 4, 18, 97, 570, 3489, 21810, 138044, 881098, 5658793, 36520068, 236619717, 1538134272, 10026366588, 65513185896, 428957025672
Offset: 0
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.