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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 40 |
| 3 | 442 |
| 4 | 5136 |
| 5 | 61424 |
| 6 | 748462 |
| 7 | 9240480 |
| 8 | 115194720 |
| 9 | 1446820588 |
| 10 | 18279806600 |
| 11 | 232071505120 |
| 12 | 2958062657550 |
| 13 | 37831613904036 |
| 14 | 485233557808704 |
| 15 | 6239148779539472 |
| 16 | 80397210629541696 |
| 17 | 1037970502613332320 |
| 18 | 13423439565585274180 |
| 19 | 173859642721737225552 |
[1,4,40,442,5136,61424,748462,9240480,115194720, 1446820588,18279806600,232071505120,2958062657550, 37831613904036,485233557808704,6239148779539472, 80397210629541696,1037970502613332320, 13423439565585274180,173859642721737225552] |