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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 88 |
| 5 | 456 |
| 6 | 2464 |
| 7 | 13736 |
| 8 | 78432 |
| 9 | 456416 |
| 10 | 2697088 |
| 11 | 16141120 |
| 12 | 97632000 |
| 13 | 595912960 |
| 14 | 3665728512 |
| 15 | 22703097472 |
| 16 | 141448381952 |
| 17 | 885934151168 |
| 18 | 5575020435456 |
| 19 | 35230798994432 |
| 20 | 223485795258368 |
| 21 | 1422572226146304 |
| 22 | 9083682419818496 |
| 23 | 58169612565614592 |
| 24 | 373486362257899520 |
| 25 | 2403850703479816192 |
[1,1,4,18,88,456,2464,13736,78432,456416,2697088, 16141120,97632000,595912960,3665728512, 22703097472,141448381952,885934151168, 5575020435456,35230798994432,223485795258368, 1422572226146304,9083682419818496, 58169612565614592,373486362257899520, 2403850703479816192] |