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| n | a(n) |
|---|---|
| 1 | 3 |
| 2 | 18 |
| 3 | 116 |
| 4 | 810 |
| 5 | 5880 |
| 6 | 44220 |
| 7 | 341484 |
| 8 | 2690010 |
| 9 | 21522228 |
| 10 | 174336264 |
| 11 | 1426403748 |
| 12 | 11767874940 |
| 13 | 97764009000 |
| 14 | 817028131140 |
| 15 | 6863037256208 |
| 16 | 57906879556410 |
| 17 | 490505340309600 |
| 18 | 4169295414140220 |
| 19 | 35548729381861332 |
| 20 | 303941636389253448 |
| 21 | 2605214026691600584 |
[3,18,116,810,5880,44220,341484,2690010,21522228, 174336264,1426403748,11767874940,97764009000, 817028131140,6863037256208,57906879556410, 490505340309600,4169295414140220, 35548729381861332,303941636389253448, 2605214026691600584] |