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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 1 |
| 3 | 2 |
| 4 | 18 |
| 5 | 104 |
| 6 | 1015 |
| 7 | 9666 |
| 8 | 116557 |
| 9 | 1504856 |
| 10 | 22300704 |
| 11 | 358916480 |
| 12 | 6373675825 |
| 13 | 122332173300 |
| 14 | 2540560235161 |
| 15 | 56558354414870 |
| 16 | 1346402030278050 |
| 17 | 34093192112537888 |
| 18 | 915570658175517151 |
| 19 | 25983157665663651150 |
| 20 | 777141557158947654637 |
| 21 | 24430880483991543481580 |
[1,0,1,2,18,104,1015,9666,116557,1504856,22300704, 358916480,6373675825,122332173300,2540560235161, 56558354414870,1346402030278050,34093192112537888, 915570658175517151,25983157665663651150, 777141557158947654637,24430880483991543481580] |