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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 54 |
| 3 | 384 |
| 4 | 2550 |
| 5 | 16506 |
| 6 | 105840 |
| 7 | 677088 |
| 8 | 4335606 |
| 9 | 27829230 |
| 10 | 179161554 |
| 11 | 1156987728 |
| 12 | 7493841264 |
| 13 | 48672149064 |
| 14 | 316920674880 |
| 15 | 2068273848384 |
| 16 | 13525486999542 |
| 17 | 88612412883030 |
| 18 | 581503640659830 |
| 19 | 3821691744347400 |
| 20 | 25150239955660050 |
| 21 | 165713382866931570 |
[1,6,54,384,2550,16506,105840,677088,4335606, 27829230,179161554,1156987728,7493841264, 48672149064,316920674880,2068273848384, 13525486999542,88612412883030,581503640659830, 3821691744347400,25150239955660050, 165713382866931570] |