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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 28 |
| 5 | 175 |
| 6 | 1368 |
| 7 | 12838 |
| 8 | 140656 |
| 9 | 1762794 |
| 10 | 24878320 |
| 11 | 390495336 |
| 12 | 6748280064 |
| 13 | 127324033824 |
| 14 | 2604355096224 |
| 15 | 57404425654080 |
| 16 | 1356401049662208 |
| 17 | 34202807058719568 |
| 18 | 916723959720053760 |
[0,0,2,6,28,175,1368,12838,140656,1762794, 24878320,390495336,6748280064,127324033824, 2604355096224,57404425654080,1356401049662208, 34202807058719568,916723959720053760] |