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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 4 |
| 5 | 21 |
| 6 | 116 |
| 7 | 748 |
| 8 | 5549 |
| 9 | 46704 |
| 10 | 439802 |
| 11 | 4581280 |
| 12 | 52302949 |
| 13 | 649428293 |
| 14 | 8713162939 |
| 15 | 125614765716 |
| 16 | 1936560971459 |
| 17 | 31791875948134 |
| 18 | 553708506096681 |
| 19 | 10197464987280554 |
| 20 | 198000778503030764 |
| 21 | 4042515894436878101 |
[1,0,0,1,4,21,116,748,5549,46704,439802,4581280, 52302949,649428293,8713162939,125614765716, 1936560971459,31791875948134,553708506096681, 10197464987280554,198000778503030764, 4042515894436878101] |