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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 106 |
| 4 | 586 |
| 5 | 3356 |
| 6 | 19728 |
| 7 | 118382 |
| 8 | 722208 |
| 9 | 4466050 |
| 10 | 27931600 |
| 11 | 176371300 |
| 12 | 1122867012 |
| 13 | 7199842666 |
| 14 | 46454345844 |
| 15 | 301384205640 |
| 16 | 1964899532794 |
| 17 | 12866563846920 |
| 18 | 84585757496444 |
| 19 | 558060746899684 |
| 20 | 3693810227983576 |
| 21 | 24521903234307786 |
| 22 | 163234951757526400 |
[1,4,20,106,586,3356,19728,118382,722208,4466050, 27931600,176371300,1122867012,7199842666, 46454345844,301384205640,1964899532794, 12866563846920,84585757496444,558060746899684, 3693810227983576,24521903234307786, 163234951757526400] |