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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 8 |
| 5 | 30 |
| 6 | 120 |
| 7 | 506 |
| 8 | 2144 |
| 9 | 9030 |
| 10 | 37920 |
| 11 | 159218 |
| 12 | 668792 |
| 13 | 2809710 |
| 14 | 11804040 |
| 15 | 49589354 |
| 16 | 208325456 |
| 17 | 875178390 |
| 18 | 3676643760 |
| 19 | 15445668962 |
| 20 | 64887621608 |
| 21 | 272594409150 |
| 22 | 1145175432600 |
| 23 | 4810908576986 |
| 24 | 20210738740544 |
| 25 | 84905783253990 |
| 26 | 356691168912960 |
| 27 | 1498467890613458 |
[0,0,2,8,30,120,506,2144,9030,37920,159218,668792, 2809710,11804040,49589354,208325456,875178390, 3676643760,15445668962,64887621608,272594409150, 1145175432600,4810908576986,20210738740544, 84905783253990,356691168912960,1498467890613458] |