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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 12 |
| 3 | 42 |
| 4 | 144 |
| 5 | 499 |
| 6 | 1728 |
| 7 | 5986 |
| 8 | 20736 |
| 9 | 71832 |
| 10 | 248832 |
| 11 | 861980 |
| 12 | 2985984 |
| 13 | 10343752 |
| 14 | 35831808 |
| 15 | 124125024 |
| 16 | 429981696 |
| 17 | 1489500288 |
| 18 | 5159780352 |
| 19 | 17874003452 |
| 20 | 61917364224 |
| 21 | 214488041414 |
| 22 | 743008370688 |
[1,4,12,42,144,499,1728,5986,20736,71832,248832, 861980,2985984,10343752,35831808,124125024, 429981696,1489500288,5159780352,17874003452, 61917364224,214488041414,743008370688] |