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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 3 |
| 3 | 9 |
| 4 | 28 |
| 5 | 90 |
| 6 | 297 |
| 7 | 1001 |
| 8 | 3432 |
| 9 | 11934 |
| 10 | 41990 |
| 11 | 149226 |
| 12 | 534888 |
| 13 | 1931540 |
| 14 | 7020405 |
| 15 | 25662825 |
| 16 | 94287120 |
| 17 | 347993910 |
| 18 | 1289624490 |
| 19 | 4796857230 |
| 20 | 17902146600 |
| 21 | 67016296620 |
| 22 | 251577050010 |
| 23 | 946844533674 |
| 24 | 3572042254128 |
| 25 | 13505406670700 |
| 26 | 51166197843852 |
| 27 | 194214400834356 |
[0,1,3,9,28,90,297,1001,3432,11934,41990,149226, 534888,1931540,7020405,25662825,94287120, 347993910,1289624490,4796857230,17902146600, 67016296620,251577050010,946844533674, 3572042254128,13505406670700,51166197843852, 194214400834356] |