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| n | a(n) |
|---|---|
| 1 | 3 |
| 2 | 20 |
| 3 | 136 |
| 4 | 928 |
| 5 | 6336 |
| 6 | 43264 |
| 7 | 295424 |
| 8 | 2017280 |
| 9 | 13774848 |
| 10 | 94060544 |
| 11 | 642285568 |
| 12 | 4385800192 |
| 13 | 29948116992 |
| 14 | 204498534400 |
| 15 | 1396403339264 |
| 16 | 9535238438912 |
| 17 | 65110680797184 |
| 18 | 444603538866176 |
| 19 | 3035942864551936 |
| 20 | 20730714605486080 |
[3,20,136,928,6336,43264,295424,2017280,13774848, 94060544,642285568,4385800192,29948116992, 204498534400,1396403339264,9535238438912, 65110680797184,444603538866176,3035942864551936, 20730714605486080] |