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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 120 |
| 4 | 836 |
| 5 | 6608 |
| 6 | 58324 |
| 7 | 568296 |
| 8 | 6060340 |
| 9 | 70245856 |
| 10 | 879937892 |
| 11 | 11853424536 |
| 12 | 170963881892 |
| 13 | 2629912684784 |
| 14 | 42995842035316 |
| 15 | 744683072665416 |
| 16 | 13624184625098644 |
| 17 | 262594854417561856 |
| 18 | 5319099368762699012 |
| 19 | 112977659152942035192 |
| 20 | 2511041582408699358980 |
[1,4,20,120,836,6608,58324,568296,6060340, 70245856,879937892,11853424536,170963881892, 2629912684784,42995842035316,744683072665416, 13624184625098644,262594854417561856, 5319099368762699012,112977659152942035192, 2511041582408699358980] |