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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 22 |
| 5 | 136 |
| 6 | 504 |
| 7 | 5136 |
| 8 | 16966 |
| 9 | 240368 |
| 10 | 752800 |
| 11 | 13660480 |
| 12 | 41492284 |
| 13 | 918172848 |
| 14 | 2734083968 |
| 15 | 71399743744 |
| 16 | 209681631814 |
| 17 | 6312095369056 |
| 18 | 18348172005888 |
| 19 | 625558677709824 |
| 20 | 1804161160185748 |
| 21 | 68713071248064560 |
[1,0,2,4,22,136,504,5136,16966,240368,752800, 13660480,41492284,918172848,2734083968, 71399743744,209681631814,6312095369056, 18348172005888,625558677709824,1804161160185748, 68713071248064560] |