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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 20 |
| 3 | 68 |
| 4 | 308 |
| 5 | 1320 |
| 6 | 5220 |
| 7 | 22652 |
| 8 | 95220 |
| 9 | 390580 |
| 10 | 1662564 |
| 11 | 6961252 |
| 12 | 28957492 |
| 13 | 122178532 |
| 14 | 511463444 |
| 15 | 2138409476 |
| 16 | 8987766644 |
| 17 | 37641997156 |
| 18 | 157656704660 |
| 19 | 661559870852 |
| 20 | 2771791087028 |
| 21 | 11615849382436 |
| 22 | 48710299475156 |
[4,20,68,308,1320,5220,22652,95220,390580,1662564, 6961252,28957492,122178532,511463444,2138409476, 8987766644,37641997156,157656704660,661559870852, 2771791087028,11615849382436,48710299475156] |