A026303 a(n) = T(3n,n), where T is the array in A026300.
1, 3, 20, 147, 1143, 9163, 74919, 621015, 5199909, 43878315, 372530781, 3178551148, 27231924300, 234113134518, 2018601251793, 17449264269081, 151169392399773
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.