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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 2 |
| 3 | 9 |
| 4 | 44 |
| 5 | 390 |
| 6 | 3474 |
| 7 | 37800 |
| 8 | 471344 |
| 9 | 6602904 |
| 10 | 103271400 |
| 11 | 1779944760 |
| 12 | 33542915592 |
| 13 | 686101244400 |
| 14 | 15139184749584 |
| 15 | 358465510133640 |
| 16 | 9066087526045440 |
| 17 | 243928110816129600 |
| 18 | 6956913949298380224 |
| 19 | 209651038286581756800 |
| 20 | 6656701196017929467520 |
| 21 | 222116657005058778103680 |
[1,0,2,9,44,390,3474,37800,471344,6602904, 103271400,1779944760,33542915592,686101244400, 15139184749584,358465510133640,9066087526045440, 243928110816129600,6956913949298380224, 209651038286581756800,6656701196017929467520, 222116657005058778103680] |