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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 6 |
| 3 | 15 |
| 4 | 280 |
| 5 | 2040 |
| 6 | 5503 |
| 7 | 113010 |
| 8 | 857532 |
| 9 | 2340127 |
| 10 | 50784920 |
| 11 | 395160694 |
| 12 | 1085939598 |
| 13 | 24341903776 |
| 14 | 192371505064 |
| 15 | 530944513322 |
| 16 | 12155425553076 |
| 17 | 97080455287086 |
| 18 | 268728663225260 |
| 19 | 6244348288426664 |
| 20 | 50251726336557552 |
[1,6,15,280,2040,5503,113010,857532,2340127, 50784920,395160694,1085939598,24341903776, 192371505064,530944513322,12155425553076, 97080455287086,268728663225260,6244348288426664, 50251726336557552] |