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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 21 |
| 3 | 126 |
| 4 | 819 |
| 5 | 5616 |
| 6 | 40014 |
| 7 | 293436 |
| 8 | 2200770 |
| 9 | 16805880 |
| 10 | 130245570 |
| 11 | 1021926780 |
| 12 | 8102419470 |
| 13 | 64819355760 |
| 14 | 522606055815 |
| 15 | 4242331511910 |
| 16 | 34645707347265 |
| 17 | 284459491903860 |
| 18 | 2346790808206845 |
| 19 | 19444838125142430 |
| 20 | 161745698950048395 |
| 21 | 1350224965148230080 |
[1,4,21,126,819,5616,40014,293436,2200770, 16805880,130245570,1021926780,8102419470, 64819355760,522606055815,4242331511910, 34645707347265,284459491903860,2346790808206845, 19444838125142430,161745698950048395, 1350224965148230080] |