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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 8 |
| 3 | 40 |
| 4 | 216 |
| 5 | 1218 |
| 6 | 7000 |
| 7 | 40560 |
| 8 | 235824 |
| 9 | 1373090 |
| 10 | 7999592 |
| 11 | 46616920 |
| 12 | 271683720 |
| 13 | 1583441442 |
| 14 | 9228858808 |
| 15 | 53789455200 |
| 16 | 313507253856 |
| 17 | 1827252574658 |
| 18 | 10650004589000 |
| 19 | 62072766255880 |
| 20 | 361786571934264 |
| 21 | 2108646614622210 |
[2,8,40,216,1218,7000,40560,235824,1373090, 7999592,46616920,271683720,1583441442,9228858808, 53789455200,313507253856,1827252574658, 10650004589000,62072766255880,361786571934264, 2108646614622210] |