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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 273 |
| 5 | 3421 |
| 6 | 52288 |
| 7 | 941578 |
| 8 | 19505545 |
| 9 | 456790123 |
| 10 | 11931215316 |
| 11 | 343871642632 |
| 12 | 10840081272265 |
| 13 | 371026432467913 |
| 14 | 13702802011918048 |
| 15 | 543154131059225686 |
| 16 | 23000016472483168305 |
| 17 | 1036227971225610466711 |
| 18 | 49492629462587441963140 |
| 19 | 2497992686980609418282548 |
| 20 | 132849300060919364474261281 |
[1,1,4,28,273,3421,52288,941578,19505545, 456790123,11931215316,343871642632,10840081272265, 371026432467913,13702802011918048, 543154131059225686,23000016472483168305, 1036227971225610466711,49492629462587441963140, 2497992686980609418282548, 132849300060919364474261281] |