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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 6 |
| 2 | 36 |
| 3 | 196 |
| 4 | 1071 |
| 5 | 5796 |
| 6 | 31395 |
| 7 | 169884 |
| 8 | 919413 |
| 9 | 4975322 |
| 10 | 26924106 |
| 11 | 145698840 |
| 12 | 788446400 |
| 13 | 4266656226 |
| 14 | 23088902733 |
| 15 | 124944995676 |
| 16 | 676136621430 |
| 17 | 3658895818470 |
| 18 | 19800020091895 |
| 19 | 107147296401684 |
| 20 | 579824822459421 |
| 21 | 3137707025200000 |
[1,6,36,196,1071,5796,31395,169884,919413,4975322, 26924106,145698840,788446400,4266656226, 23088902733,124944995676,676136621430, 3658895818470,19800020091895,107147296401684, 579824822459421,3137707025200000] |