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| n | a(n) |
|---|---|
| -1 | 1 |
| 0 | 4 |
| 1 | 4 |
| 2 | 16 |
| 3 | 100 |
| 4 | 784 |
| 5 | 7056 |
| 6 | 69696 |
| 7 | 736164 |
| 8 | 8179600 |
| 9 | 94556176 |
| 10 | 1128422464 |
| 11 | 13823175184 |
| 12 | 173075968576 |
| 13 | 2207601640000 |
| 14 | 28610517254400 |
| 15 | 375960078296100 |
| 16 | 5000659311315600 |
| 17 | 67231086296576400 |
| 18 | 912554910950760000 |
| 19 | 12492876730915904400 |
| 20 | 172350707553043905600 |
[1,4,4,16,100,784,7056,69696,736164,8179600, 94556176,1128422464,13823175184,173075968576, 2207601640000,28610517254400,375960078296100, 5000659311315600,67231086296576400, 912554910950760000,12492876730915904400, 172350707553043905600] |