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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 80 |
| 4 | 448 |
| 5 | 2672 |
| 6 | 16640 |
| 7 | 106944 |
| 8 | 704000 |
| 9 | 4722608 |
| 10 | 32166784 |
| 11 | 221865280 |
| 12 | 1546491904 |
| 13 | 10876777024 |
| 14 | 77091573760 |
| 15 | 550088739584 |
| 16 | 3948410757120 |
| 17 | 28489277352112 |
| 18 | 206520803651712 |
| 19 | 1503353875355200 |
| 20 | 10984898330047488 |
| 21 | 80540719266134080 |
| 22 | 592362120108263424 |
| 23 | 4369140213882013440 |
[1,4,16,80,448,2672,16640,106944,704000,4722608, 32166784,221865280,1546491904,10876777024, 77091573760,550088739584,3948410757120, 28489277352112,206520803651712,1503353875355200, 10984898330047488,80540719266134080, 592362120108263424,4369140213882013440] |