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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 80 |
| 5 | 471 |
| 6 | 3127 |
| 7 | 23059 |
| 8 | 186468 |
| 9 | 1635265 |
| 10 | 15422471 |
| 11 | 155388399 |
| 12 | 1663294756 |
| 13 | 18826525771 |
| 14 | 224434810797 |
| 15 | 2808247979611 |
| 16 | 36770685485408 |
| 17 | 502505495269521 |
| 18 | 7150461569849395 |
| 19 | 105723461155720879 |
| 20 | 1621191824611307436 |
| 21 | 25738508587975433251 |
[1,1,4,16,80,471,3127,23059,186468,1635265, 15422471,155388399,1663294756,18826525771, 224434810797,2808247979611,36770685485408, 502505495269521,7150461569849395, 105723461155720879,1621191824611307436, 25738508587975433251] |