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) |
|---|---|
| 1 | 4 |
| 2 | -10 |
| 3 | 20 |
| 4 | -60 |
| 5 | 204 |
| 6 | -690 |
| 7 | 2340 |
| 8 | -8160 |
| 9 | 29120 |
| 10 | -104958 |
| 11 | 381300 |
| 12 | -1397740 |
| 13 | 5162220 |
| 14 | -19175130 |
| 15 | 71582716 |
| 16 | -268431360 |
| 17 | 1010580540 |
| 18 | -3817763040 |
| 19 | 14467258260 |
| 20 | -54975528948 |
| 21 | 209430785460 |
| 22 | -799645010850 |
| 23 | 3059510616420 |