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 | 2 |
| 2 | 6 |
| 3 | 18 |
| 4 | 64 |
| 5 | 220 |
| 6 | 888 |
| 7 | 3192 |
| 8 | 15104 |
| 9 | 48096 |
| 10 | 338400 |
| 11 | 285120 |
| 12 | 13728768 |
| 13 | -75484032 |
| 14 | 1327431168 |
| 15 | -15621137280 |
| 16 | 232048220160 |
| 17 | -3468200017920 |
| 18 | 56208508250112 |
| 19 | -959557688285184 |
| 20 | 17344153658234880 |
| 21 | -330228975428997120 |
| 22 | 6611419866845122560 |
| 23 | -138844103855232061440 |