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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 6 |
| 3 | 16 |
| 4 | 45 |
| 5 | 144 |
| 6 | 440 |
| 7 | 1440 |
| 8 | 4680 |
| 9 | 15600 |
| 10 | 52344 |
| 11 | 177840 |
| 12 | 608160 |
| 13 | 2095920 |
| 14 | 7262640 |
| 15 | 25300032 |
| 16 | 88517520 |
| 17 | 310927680 |
| 18 | 1095923400 |
| 19 | 3874804560 |
| 20 | 13737892896 |
| 21 | 48829153920 |
| 22 | 173949483240 |
| 23 | 620963048160 |
| 24 | 2220904271040 |
| 25 | 7956987570576 |
| 26 | 28553731537320 |
| 27 | 102617166646800 |
[1,4,6,16,45,144,440,1440,4680,15600,52344,177840, 608160,2095920,7262640,25300032,88517520, 310927680,1095923400,3874804560,13737892896, 48829153920,173949483240,620963048160, 2220904271040,7956987570576,28553731537320, 102617166646800] |