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 | 2 |
| 3 | -6 |
| 4 | -72 |
| 5 | -520 |
| 6 | -1200 |
| 7 | 24752 |
| 8 | 516992 |
| 9 | 5106816 |
| 10 | 5287680 |
| 11 | -998945024 |
| 12 | -23719719936 |
| 13 | -272471972864 |
| 14 | 1326261594112 |
| 15 | 149170761246720 |
| 16 | 3843177252618240 |
| 17 | 42752553478356992 |
| 18 | -863092250325614592 |
| 19 | -59317347865870139392 |
| 20 | -1577115871098630307840 |
| 21 | -13173264127625587851264 |