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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 16 |
| 3 | 60 |
| 4 | 220 |
| 5 | 788 |
| 6 | 2780 |
| 7 | 9684 |
| 8 | 33404 |
| 9 | 114292 |
| 10 | 388444 |
| 11 | 1312788 |
| 12 | 4415548 |
| 13 | 14790836 |
| 14 | 49369628 |
| 15 | 164279892 |
| 16 | 545170172 |
| 17 | 1804855540 |
| 18 | 5962578652 |
| 19 | 19661140116 |
| 20 | 64722276796 |
| 21 | 212738159924 |
| 22 | 698312564636 |
| 23 | 2289419181780 |
| 24 | 7497612860540 |
[4,16,60,220,788,2780,9684,33404,114292,388444, 1312788,4415548,14790836,49369628,164279892, 545170172,1804855540,5962578652,19661140116, 64722276796,212738159924,698312564636, 2289419181780,7497612860540] |