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A220457 as a simple table
| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 22 |
| 3 | 108 |
| 4 | 691 |
| 5 | 3872 |
| 6 | 20897 |
| 7 | 112582 |
| 8 | 605780 |
| 9 | 3249132 |
| 10 | 17400206 |
| 11 | 93125105 |
| 12 | 498148634 |
| 13 | 2663790130 |
| 14 | 14241169579 |
| 15 | 76125716517 |
| 16 | 406890173406 |
| 17 | 2174685548474 |
| 18 | 11622471918192 |
| 19 | 62113961444133 |
| 20 | 331949857443568 |
| 21 | 1773988622548460 |
[4,22,108,691,3872,20897,112582,605780,3249132, 17400206,93125105,498148634,2663790130, 14241169579,76125716517,406890173406, 2174685548474,11622471918192,62113961444133, 331949857443568,1773988622548460] |