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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 40 |
| 3 | 448 |
| 4 | 5264 |
| 5 | 63624 |
| 6 | 783328 |
| 7 | 9770240 |
| 8 | 123040288 |
| 9 | 1561033348 |
| 10 | 19922193200 |
| 11 | 255472920256 |
| 12 | 3289122824000 |
| 13 | 42488488508808 |
| 14 | 550435283089088 |
| 15 | 7148519205631488 |
| 16 | 93038785849116736 |
| 17 | 1213215382135324680 |
| 18 | 15846906866928513736 |
| 19 | 207302985358274247104 |
[1,4,40,448,5264,63624,783328,9770240,123040288, 1561033348,19922193200,255472920256,3289122824000, 42488488508808,550435283089088,7148519205631488, 93038785849116736,1213215382135324680, 15846906866928513736,207302985358274247104] |