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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 112 |
| 4 | 736 |
| 5 | 5632 |
| 6 | 49024 |
| 7 | 474112 |
| 8 | 5017600 |
| 9 | 57597952 |
| 10 | 712597504 |
| 11 | 9446981632 |
| 12 | 133474877440 |
| 13 | 2000265674752 |
| 14 | 31666683510784 |
| 15 | 527786775150592 |
| 16 | 9233419259084800 |
| 17 | 169106747636580352 |
| 18 | 3234542505882025984 |
| 19 | 64473076850860490752 |
| 20 | 1336621867385969704960 |
| 21 | 28769619371258703511552 |
[1,4,20,112,736,5632,49024,474112,5017600, 57597952,712597504,9446981632,133474877440, 2000265674752,31666683510784,527786775150592, 9233419259084800,169106747636580352, 3234542505882025984,64473076850860490752, 1336621867385969704960,28769619371258703511552] |