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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 6 |
| 3 | 18 |
| 4 | 54 |
| 5 | 172 |
| 6 | 564 |
| 7 | 1900 |
| 8 | 6542 |
| 9 | 23000 |
| 10 | 82025 |
| 11 | 295947 |
| 12 | 1077871 |
| 13 | 3957809 |
| 14 | 14630843 |
| 15 | 54400028 |
| 16 | 203280221 |
| 17 | 762939111 |
| 18 | 2874398515 |
| 19 | 10866266172 |
| 20 | 41203088796 |
| 21 | 156661034233 |
| 22 | 597116381732 |
| 23 | 2280998753949 |
| 24 | 8731188863470 |
| 25 | 33483379603407 |
| 26 | 128625503610475 |
| 27 | 494890204904784 |
[0,2,6,18,54,172,564,1900,6542,23000,82025,295947, 1077871,3957809,14630843,54400028,203280221, 762939111,2874398515,10866266172,41203088796, 156661034233,597116381732,2280998753949, 8731188863470,33483379603407,128625503610475, 494890204904784] |