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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 196 |
| 4 | 1324 |
| 5 | 8980 |
| 6 | 60028 |
| 7 | 402412 |
| 8 | 2675860 |
| 9 | 17826340 |
| 10 | 118145548 |
| 11 | 784024780 |
| 12 | 5184334996 |
| 13 | 34313323804 |
| 14 | 226516271020 |
| 15 | 1496391824212 |
| 16 | 9865667928796 |
| 17 | 65080520041804 |
| 18 | 428641139406628 |
| 19 | 2824446024265444 |
| 20 | 18587519784608836 |
| 21 | 122369125319060884 |
[1,4,28,196,1324,8980,60028,402412,2675860, 17826340,118145548,784024780,5184334996, 34313323804,226516271020,1496391824212, 9865667928796,65080520041804,428641139406628, 2824446024265444,18587519784608836, 122369125319060884] |