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 | 9 |
| 4 | 44 |
| 5 | 390 |
| 6 | 2754 |
| 7 | 32760 |
| 8 | 310064 |
| 9 | 4244184 |
| 10 | 54887400 |
| 11 | 818615160 |
| 12 | 12909921672 |
| 13 | 225872515440 |
| 14 | 4045885572624 |
| 15 | 79360837887240 |
| 16 | 1649832369335040 |
| 17 | 35666417240193600 |
| 18 | 822291935260976064 |
| 19 | 19830352438530840960 |
| 20 | 501144432316767688320 |
| 21 | 13229590606682042436480 |