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| n | a(n) |
|---|---|
| 0 | 4 |
| 1 | 20 |
| 2 | 130 |
| 3 | 884 |
| 4 | 6052 |
| 5 | 41474 |
| 6 | 284260 |
| 7 | 1948340 |
| 8 | 13354114 |
| 9 | 91530452 |
| 10 | 627359044 |
| 11 | 4299982850 |
| 12 | 29472520900 |
| 13 | 202007663444 |
| 14 | 1384581123202 |
| 15 | 9490060198964 |
| 16 | 65045840269540 |
| 17 | 445830821687810 |
| 18 | 3055769911545124 |
| 19 | 20944558559128052 |
| 20 | 143556140002351234 |
| 21 | 983948421457330580 |
[4,20,130,884,6052,41474,284260,1948340,13354114, 91530452,627359044,4299982850,29472520900, 202007663444,1384581123202,9490060198964, 65045840269540,445830821687810,3055769911545124, 20944558559128052,143556140002351234, 983948421457330580] |