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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 116 |
| 4 | 720 |
| 5 | 4656 |
| 6 | 30996 |
| 7 | 210896 |
| 8 | 1459536 |
| 9 | 10239796 |
| 10 | 72651184 |
| 11 | 520328112 |
| 12 | 3756512912 |
| 13 | 27307671040 |
| 14 | 199705789248 |
| 15 | 1468209751856 |
| 16 | 10844681408064 |
| 17 | 80437588353600 |
| 18 | 598867568439828 |
| 19 | 4473784063109904 |
| 20 | 33524058847464912 |
[1,4,20,116,720,4656,30996,210896,1459536, 10239796,72651184,520328112,3756512912, 27307671040,199705789248,1468209751856, 10844681408064,80437588353600,598867568439828, 4473784063109904,33524058847464912] |