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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 24 |
| 5 | 180 |
| 6 | 1240 |
| 7 | 10456 |
| 8 | 112896 |
| 9 | 1316816 |
| 10 | 16378528 |
| 11 | 229634304 |
| 12 | 3569726336 |
| 13 | 59315089600 |
| 14 | 1055150016128 |
| 15 | 20254767489152 |
| 16 | 415619313491968 |
| 17 | 9028238025838848 |
| 18 | 207506148418183680 |
| 19 | 5043727679716764672 |
[0,1,2,4,24,180,1240,10456,112896,1316816, 16378528,229634304,3569726336,59315089600, 1055150016128,20254767489152,415619313491968, 9028238025838848,207506148418183680, 5043727679716764672] |