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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 6 |
| 3 | 16 |
| 4 | 106 |
| 5 | 426 |
| 6 | 3076 |
| 7 | 15856 |
| 8 | 123516 |
| 9 | 757756 |
| 10 | 6315976 |
| 11 | 44203776 |
| 12 | 391582456 |
| 13 | 3043809016 |
| 14 | 28496668656 |
| 15 | 241563299776 |
| 16 | 2378813448976 |
| 17 | 21703877431056 |
| 18 | 223903020594016 |
| 19 | 2177251989389056 |
| 20 | 23448038945820576 |
| 21 | 241173237884726176 |
[1,1,6,16,106,426,3076,15856,123516,757756, 6315976,44203776,391582456,3043809016,28496668656, 241563299776,2378813448976,21703877431056, 223903020594016,2177251989389056, 23448038945820576,241173237884726176] |