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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 6 |
| 5 | 26 |
| 6 | 78 |
| 7 | 288 |
| 8 | 888 |
| 9 | 3130 |
| 10 | 9850 |
| 11 | 34112 |
| 12 | 108998 |
| 13 | 374636 |
| 14 | 1211046 |
| 15 | 4148816 |
| 16 | 13533796 |
| 17 | 46304730 |
| 18 | 152153758 |
| 19 | 520434552 |
| 20 | 1720325302 |
| 21 | 5885686496 |
| 22 | 19552190624 |
| 23 | 66927118548 |
| 24 | 223264746520 |
| 25 | 764725528072 |
| 26 | 2560239468774 |
| 27 | 8775478294368 |
| 28 | 29470844083770 |
[0,0,2,6,26,78,288,888,3130,9850,34112,108998, 374636,1211046,4148816,13533796,46304730, 152153758,520434552,1720325302,5885686496, 19552190624,66927118548,223264746520,764725528072, 2560239468774,8775478294368,29470844083770] |