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 | 0 |
| 2 | 2 |
| 3 | 12 |
| 4 | 72 |
| 5 | 520 |
| 6 | 5040 |
| 7 | 67284 |
| 8 | 1156736 |
| 9 | 23655888 |
| 10 | 549676800 |
| 11 | 14216252380 |
| 12 | 405068387328 |
| 13 | 12624364306008 |
| 14 | 427599019108352 |
| 15 | 15646376279614500 |
| 16 | 615155126821355520 |
| 17 | 25861820048469628576 |
| 18 | 1157706908035331457024 |
| 19 | 54977324662490442177708 |
| 20 | 2760439046217459138560000 |