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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 6 |
| 3 | 40 |
| 4 | 350 |
| 5 | 4032 |
| 6 | 56364 |
| 7 | 933504 |
| 8 | 17824950 |
| 9 | 385848320 |
| 10 | 9334057876 |
| 11 | 249576198144 |
| 12 | 7308698191340 |
| 13 | 232643283353600 |
| 14 | 7997684730384600 |
| 15 | 295306112919306240 |
| 16 | 11655857682806336550 |
| 17 | 489743069731226910720 |
| 18 | 21824608434847162167300 |
| 19 | 1028154317960939805081600 |
[0,2,6,40,350,4032,56364,933504,17824950, 385848320,9334057876,249576198144,7308698191340, 232643283353600,7997684730384600, 295306112919306240,11655857682806336550, 489743069731226910720,21824608434847162167300, 1028154317960939805081600] |