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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 22 |
| 4 | 112 |
| 5 | 556 |
| 6 | 2704 |
| 7 | 13000 |
| 8 | 62080 |
| 9 | 295312 |
| 10 | 1401664 |
| 11 | 6644320 |
| 12 | 31472896 |
| 13 | 149017792 |
| 14 | 705395968 |
| 15 | 3338614912 |
| 16 | 15800258560 |
| 17 | 74772443392 |
| 18 | 353840161792 |
| 19 | 1674425579008 |
| 20 | 7923565146112 |
| 21 | 37494981225472 |
| 22 | 177428889407488 |
[0,1,4,22,112,556,2704,13000,62080,295312,1401664, 6644320,31472896,149017792,705395968,3338614912, 15800258560,74772443392,353840161792, 1674425579008,7923565146112,37494981225472, 177428889407488] |