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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 24 |
| 3 | 206 |
| 4 | 1322 |
| 5 | 9970 |
| 6 | 63892 |
| 7 | 459420 |
| 8 | 2983714 |
| 9 | 21011004 |
| 10 | 138067258 |
| 11 | 960196414 |
| 12 | 6366988610 |
| 13 | 43918588986 |
| 14 | 293115852448 |
| 15 | 2010639717594 |
| 16 | 13480363485774 |
| 17 | 92114886446716 |
| 18 | 619546400267578 |
| 19 | 4222299248961016 |
| 20 | 28460962259793426 |
[4,24,206,1322,9970,63892,459420,2983714,21011004, 138067258,960196414,6366988610,43918588986, 293115852448,2010639717594,13480363485774, 92114886446716,619546400267578,4222299248961016, 28460962259793426] |