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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 4 |
| 5 | 20 |
| 6 | 90 |
| 7 | 455 |
| 8 | 2352 |
| 9 | 13132 |
| 10 | 76540 |
| 11 | 473660 |
| 12 | 3069220 |
| 13 | 20922330 |
| 14 | 149021600 |
| 15 | 1109629885 |
| 16 | 8604815520 |
| 17 | 69437698160 |
| 18 | 581661169640 |
| 19 | 5051885815603 |
| 20 | 45411759404560 |
| 21 | 421977921782270 |
| 22 | 4047693372023070 |
| 23 | 40034523497947132 |
| 24 | 407818256494533984 |
| 25 | 4274309903558446900 |
[0,0,0,1,4,20,90,455,2352,13132,76540,473660, 3069220,20922330,149021600,1109629885,8604815520, 69437698160,581661169640,5051885815603, 45411759404560,421977921782270,4047693372023070, 40034523497947132,407818256494533984, 4274309903558446900] |