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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 124 |
| 4 | 936 |
| 5 | 8424 |
| 6 | 88648 |
| 7 | 1072432 |
| 8 | 14702720 |
| 9 | 225692128 |
| 10 | 3839770656 |
| 11 | 71780577312 |
| 12 | 1463532416320 |
| 13 | 32337850727680 |
| 14 | 770039603953664 |
| 15 | 19664621381714944 |
| 16 | 536234348295180288 |
| 17 | 15554459021934423552 |
| 18 | 478297493455731968512 |
| 19 | 15543431292269887979008 |
[1,4,20,124,936,8424,88648,1072432,14702720, 225692128,3839770656,71780577312,1463532416320, 32337850727680,770039603953664,19664621381714944, 536234348295180288,15554459021934423552, 478297493455731968512,15543431292269887979008] |