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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 18 |
| 5 | 84 |
| 6 | 405 |
| 7 | 2002 |
| 8 | 10101 |
| 9 | 51844 |
| 10 | 269994 |
| 11 | 1423784 |
| 12 | 7590044 |
| 13 | 40846390 |
| 14 | 221650195 |
| 15 | 1211606190 |
[0,1,4,18,84,405,2002,10101,51844,269994,1423784, 7590044,40846390,221650195,1211606190] |