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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 4 |
| 3 | 132 |
| 4 | 256 |
| 5 | 5260 |
| 6 | 37636 |
| 7 | 321696 |
| 8 | 2951524 |
| 9 | 26486820 |
| 10 | 229461904 |
| 11 | 2051332140 |
| 12 | 18106931844 |
| 13 | 160285145184 |
| 14 | 1419610292676 |
| 15 | 12571883432500 |
| 16 | 111281907352576 |
| 17 | 985420995488860 |
| 18 | 8724655803209956 |
| 19 | 77248992679400736 |
| 20 | 683971842578913604 |
[0,4,132,256,5260,37636,321696,2951524,26486820, 229461904,2051332140,18106931844,160285145184, 1419610292676,12571883432500,111281907352576, 985420995488860,8724655803209956, 77248992679400736,683971842578913604] |