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| n | a(n) |
|---|---|
| 0 | 4 |
| 1 | 16 |
| 2 | 88 |
| 3 | 520 |
| 4 | 3112 |
| 5 | 18664 |
| 6 | 111976 |
| 7 | 671848 |
| 8 | 4031080 |
| 9 | 24186472 |
| 10 | 145118824 |
| 11 | 870712936 |
| 12 | 5224277608 |
| 13 | 31345665640 |
| 14 | 188073993832 |
| 15 | 1128443962984 |
| 16 | 6770663777896 |
| 17 | 40623982667368 |
| 18 | 243743896004200 |
| 19 | 1462463376025192 |
[4,16,88,520,3112,18664,111976,671848,4031080, 24186472,145118824,870712936,5224277608, 31345665640,188073993832,1128443962984, 6770663777896,40623982667368,243743896004200, 1462463376025192] |