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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 65 |
| 4 | 272 |
| 5 | 1184 |
| 6 | 5376 |
| 7 | 25344 |
| 8 | 122880 |
| 9 | 606208 |
| 10 | 3014656 |
| 11 | 15007744 |
| 12 | 74448896 |
| 13 | 367001600 |
| 14 | 1795162112 |
| 15 | 8707375104 |
| 16 | 41875931136 |
| 17 | 199715979264 |
| 18 | 944892805120 |
| 19 | 4436701216768 |
| 20 | 20684562497536 |
| 21 | 95794950569984 |
| 22 | 440904162738176 |
[1,4,16,65,272,1184,5376,25344,122880,606208, 3014656,15007744,74448896,367001600,1795162112, 8707375104,41875931136,199715979264,944892805120, 4436701216768,20684562497536,95794950569984, 440904162738176] |