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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 21 |
| 3 | 120 |
| 4 | 715 |
| 5 | 4368 |
| 6 | 27132 |
| 7 | 170544 |
| 8 | 1081575 |
| 9 | 6906900 |
| 10 | 44352165 |
| 11 | 286097760 |
| 12 | 1852482996 |
| 13 | 12033222880 |
| 14 | 78378960360 |
| 15 | 511738760544 |
| 16 | 3348108992991 |
| 17 | 21945588357420 |
| 18 | 144079707346575 |
| 19 | 947309492837400 |
| 20 | 6236646703759395 |
[1,4,21,120,715,4368,27132,170544,1081575,6906900, 44352165,286097760,1852482996,12033222880, 78378960360,511738760544,3348108992991, 21945588357420,144079707346575,947309492837400, 6236646703759395] |