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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 72 |
| 5 | 275 |
| 6 | 1093 |
| 7 | 4479 |
| 8 | 18801 |
| 9 | 80451 |
| 10 | 349700 |
| 11 | 1540012 |
| 12 | 6856998 |
| 13 | 30819381 |
| 14 | 139647086 |
| 15 | 637238709 |
| 16 | 2925903993 |
| 17 | 13508063971 |
| 18 | 62666934328 |
| 19 | 291993695249 |
| 20 | 1365865078510 |
| 21 | 6411761519748 |
| 22 | 30195341581719 |
| 23 | 142617094236380 |
| 24 | 675403835836741 |
| 25 | 3206416193453260 |
| 26 | 15256531432328357 |
| 27 | 72743764252082951 |
[1,2,6,20,72,275,1093,4479,18801,80451,349700, 1540012,6856998,30819381,139647086,637238709, 2925903993,13508063971,62666934328,291993695249, 1365865078510,6411761519748,30195341581719, 142617094236380,675403835836741,3206416193453260, 15256531432328357,72743764252082951] |