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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 4 |
| 3 | 20 |
| 4 | 111 |
| 5 | 616 |
| 6 | 3465 |
| 7 | 19668 |
| 8 | 112476 |
| 9 | 647088 |
| 10 | 3741156 |
| 11 | 21718520 |
| 12 | 126520240 |
| 13 | 739225704 |
| 14 | 4330188265 |
| 15 | 25421864580 |
| 16 | 149541718275 |
| 17 | 881200519200 |
| 18 | 5200715525016 |
| 19 | 30736780735752 |
| 20 | 181886951796886 |
[0,4,20,111,616,3465,19668,112476,647088,3741156, 21718520,126520240,739225704,4330188265, 25421864580,149541718275,881200519200, 5200715525016,30736780735752,181886951796886] |