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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 113 |
| 5 | 688 |
| 6 | 4404 |
| 7 | 29219 |
| 8 | 199140 |
| 9 | 1385904 |
| 10 | 9807820 |
| 11 | 70364704 |
| 12 | 510609620 |
| 13 | 3741212535 |
| 14 | 27639233548 |
| 15 | 205660399220 |
| 16 | 1539916433473 |
| 17 | 11594310041792 |
| 18 | 87725707127600 |
| 19 | 666681174728724 |
| 20 | 5086601816592432 |
| 21 | 38948589882247968 |
[1,1,4,20,113,688,4404,29219,199140,1385904, 9807820,70364704,510609620,3741212535,27639233548, 205660399220,1539916433473,11594310041792, 87725707127600,666681174728724,5086601816592432, 38948589882247968] |