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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 4 |
| 5 | 12 |
| 6 | 40 |
| 7 | 144 |
| 8 | 512 |
| 9 | 1813 |
| 10 | 6528 |
| 11 | 23808 |
| 12 | 87380 |
| 13 | 322560 |
| 14 | 1198080 |
| 15 | 4473647 |
| 16 | 16777216 |
| 17 | 63160320 |
| 18 | 238605640 |
| 19 | 904200192 |
| 20 | 3435973836 |
| 21 | 13089411609 |
| 22 | 49977753600 |
| 23 | 191219367936 |
| 24 | 733007751680 |
| 25 | 2814749599332 |
| 26 | 10825959997440 |
| 27 | 41699995927744 |
| 28 | 160842843834660 |
| 29 | 621186153185280 |
[0,0,1,4,12,40,144,512,1813,6528,23808,87380, 322560,1198080,4473647,16777216,63160320, 238605640,904200192,3435973836,13089411609, 49977753600,191219367936,733007751680, 2814749599332,10825959997440,41699995927744, 160842843834660,621186153185280] |