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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 6 |
| 3 | 24 |
| 4 | 136 |
| 5 | 726 |
| 6 | 4050 |
| 7 | 22786 |
| 8 | 130453 |
| 9 | 755434 |
| 10 | 4423012 |
| 11 | 26136264 |
| 12 | 155721334 |
| 13 | 934541838 |
| 14 | 5644979312 |
| 15 | 34296099090 |
| 16 | 209459048259 |
| 17 | 1285320713656 |
| 18 | 7921311099098 |
| 19 | 49011099602408 |
| 20 | 304342233975052 |
| 21 | 1896163114078450 |
[0,6,24,136,726,4050,22786,130453,755434,4423012, 26136264,155721334,934541838,5644979312, 34296099090,209459048259,1285320713656, 7921311099098,49011099602408,304342233975052, 1896163114078450] |