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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 10 |
| 4 | 36 |
| 5 | 116 |
| 6 | 400 |
| 7 | 1352 |
| 8 | 4624 |
| 9 | 15760 |
| 10 | 53824 |
| 11 | 183712 |
| 12 | 627264 |
| 13 | 2141504 |
| 14 | 7311616 |
| 15 | 24963200 |
| 16 | 85229824 |
| 17 | 290992384 |
| 18 | 993510400 |
| 19 | 3392055808 |
| 20 | 11581203456 |
| 21 | 39540700160 |
| 22 | 135000395776 |
| 23 | 460920178688 |
| 24 | 1573679927296 |
| 25 | 5372879343616 |
[1,1,4,10,36,116,400,1352,4624,15760,53824,183712, 627264,2141504,7311616,24963200,85229824, 290992384,993510400,3392055808,11581203456, 39540700160,135000395776,460920178688, 1573679927296,5372879343616] |