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.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | -1 |
| 2 | 0 |
| 3 | 2 |
| 4 | 11 |
| 5 | 97 |
| 6 | 1105 |
| 7 | 13905 |
| 8 | 189633 |
| 9 | 2803873 |
| 10 | 44875599 |
| 11 | 774931369 |
| 12 | 14385283720 |
| 13 | 286010393240 |
| 14 | 6069158797378 |
| 15 | 137001560841500 |
| 16 | 3279733485742038 |
| 17 | 83029219746406610 |
| 18 | 2216965307395350215 |
| 19 | 62282348739259200015 |
| 20 | 1836840301301564071196 |
| 21 | 56750954571528670989664 |