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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 16 |
| 3 | 54 |
| 4 | 166 |
| 5 | 500 |
| 6 | 1557 |
| 7 | 5316 |
| 8 | 19292 |
| 9 | 69536 |
| 10 | 248613 |
| 11 | 893091 |
| 12 | 3146780 |
| 13 | 10584619 |
| 14 | 34064054 |
| 15 | 108011322 |
| 16 | 350399707 |
| 17 | 1186298040 |
| 18 | 4158109604 |
| 19 | 14813200901 |
| 20 | 52859389574 |
| 21 | 186599450347 |
| 22 | 643953634665 |
| 23 | 2160180481888 |
| 24 | 7086155229600 |
[4,16,54,166,500,1557,5316,19292,69536,248613, 893091,3146780,10584619,34064054,108011322, 350399707,1186298040,4158109604,14813200901, 52859389574,186599450347,643953634665, 2160180481888,7086155229600] |