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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 60 |
| 5 | 200 |
| 6 | 624 |
| 7 | 1911 |
| 8 | 5712 |
| 9 | 16830 |
| 10 | 48950 |
| 11 | 140976 |
| 12 | 402624 |
| 13 | 1141933 |
| 14 | 3219580 |
| 15 | 9031050 |
| 16 | 25219824 |
| 17 | 70153016 |
| 18 | 194466672 |
| 19 | 537404835 |
| 20 | 1480993800 |
| 21 | 4071156726 |
| 22 | 11165970794 |
| 23 | 30561658848 |
| 24 | 83490220800 |
| 25 | 227687745625 |
| 26 | 619938027124 |
| 27 | 1685442626946 |
| 28 | 4575973716132 |
[0,1,4,18,60,200,624,1911,5712,16830,48950,140976, 402624,1141933,3219580,9031050,25219824,70153016, 194466672,537404835,1480993800,4071156726, 11165970794,30561658848,83490220800,227687745625, 619938027124,1685442626946,4575973716132] |