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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 23 |
| 3 | 153 |
| 4 | 1118 |
| 5 | 8772 |
| 6 | 71918 |
| 7 | 609237 |
| 8 | 5286238 |
| 9 | 46697909 |
| 10 | 418348044 |
| 11 | 3790060378 |
| 12 | 34652117969 |
| 13 | 319239995375 |
| 14 | 2960006060823 |
| 15 | 27596305747873 |
[1,4,23,153,1118,8772,71918,609237,5286238, 46697909,418348044,3790060378,34652117969, 319239995375,2960006060823,27596305747873] |