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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 18 |
| 3 | 88 |
| 4 | 460 |
| 5 | 2544 |
| 6 | 14776 |
| 7 | 89632 |
| 8 | 565392 |
| 9 | 3695680 |
| 10 | 24959776 |
| 11 | 173752704 |
| 12 | 1244125888 |
| 13 | 9146568448 |
| 14 | 68933546880 |
| 15 | 531838104064 |
| 16 | 4195358822656 |
| 17 | 33800254620672 |
| 18 | 277843218452992 |
| 19 | 2328182040156160 |
| 20 | 19870770461838336 |
| 21 | 172610363453599744 |
| 22 | 1525013813211609088 |
[4,18,88,460,2544,14776,89632,565392,3695680, 24959776,173752704,1244125888,9146568448, 68933546880,531838104064,4195358822656, 33800254620672,277843218452992,2328182040156160, 19870770461838336,172610363453599744, 1525013813211609088] |