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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 6 |
| 5 | 24 |
| 6 | 84 |
| 7 | 312 |
| 8 | 1140 |
| 9 | 4248 |
| 10 | 15852 |
| 11 | 59688 |
| 12 | 225624 |
| 13 | 857424 |
| 14 | 3270216 |
| 15 | 12518472 |
| 16 | 48062520 |
| 17 | 185034360 |
| 18 | 714038448 |
| 19 | 2761343064 |
| 20 | 10698992304 |
| 21 | 41525207448 |
| 22 | 161418403008 |
| 23 | 628356425856 |
| 24 | 2449152822168 |
| 25 | 9557303840184 |
[0,0,2,6,24,84,312,1140,4248,15852,59688,225624, 857424,3270216,12518472,48062520,185034360, 714038448,2761343064,10698992304,41525207448, 161418403008,628356425856,2449152822168, 9557303840184] |