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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 18 |
| 3 | 81 |
| 4 | 364 |
| 5 | 1636 |
| 6 | 7353 |
| 7 | 33048 |
| 8 | 148534 |
| 9 | 667585 |
| 10 | 3000456 |
| 11 | 13485528 |
| 12 | 60610609 |
| 13 | 272413948 |
| 14 | 1224362538 |
| 15 | 5502888657 |
| 16 | 24732693652 |
| 17 | 111160914460 |
| 18 | 499611933801 |
| 19 | 2245502257776 |
| 20 | 10092393813166 |
| 21 | 45360191702017 |
| 22 | 203871056692176 |
[4,18,81,364,1636,7353,33048,148534,667585, 3000456,13485528,60610609,272413948,1224362538, 5502888657,24732693652,111160914460,499611933801, 2245502257776,10092393813166,45360191702017, 203871056692176] |