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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 2 |
| 5 | 6 |
| 6 | 16 |
| 7 | 54 |
| 8 | 154 |
| 9 | 480 |
| 10 | 1418 |
| 11 | 4302 |
| 12 | 12836 |
| 13 | 38670 |
| 14 | 115802 |
| 15 | 347868 |
| 16 | 1042970 |
| 17 | 3130350 |
| 18 | 9389152 |
| 19 | 28171710 |
| 20 | 84509410 |
| 21 | 253541136 |
| 22 | 760606270 |
| 23 | 2281857318 |
| 24 | 6845520448 |
| 25 | 20536677354 |
| 26 | 61609877590 |
| 27 | 184829980176 |
| 28 | 554489476858 |
| 29 | 1663469474178 |
| 30 | 4990407031696 |
[0,0,0,2,6,16,54,154,480,1418,4302,12836,38670, 115802,347868,1042970,3130350,9389152,28171710, 84509410,253541136,760606270,2281857318, 6845520448,20536677354,61609877590,184829980176, 554489476858,1663469474178,4990407031696] |