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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 80 |
| 4 | 280 |
| 5 | 896 |
| 6 | 2688 |
| 7 | 7680 |
| 8 | 21120 |
| 9 | 56320 |
| 10 | 146432 |
| 11 | 372736 |
| 12 | 931840 |
| 13 | 2293760 |
| 14 | 5570560 |
| 15 | 13369344 |
| 16 | 31752192 |
| 17 | 74711040 |
| 18 | 174325760 |
| 19 | 403701760 |
| 20 | 928514048 |
| 21 | 2122317824 |
| 22 | 4823449600 |
| 23 | 10905190400 |
| 24 | 24536678400 |
| 25 | 54962159616 |
| 26 | 122607894528 |
[1,4,20,80,280,896,2688,7680,21120,56320,146432, 372736,931840,2293760,5570560,13369344,31752192, 74711040,174325760,403701760,928514048,2122317824, 4823449600,10905190400,24536678400,54962159616, 122607894528] |