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A000307 as a simple table
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 22 |
| 4 | 154 |
| 5 | 1304 |
| 6 | 12915 |
| 7 | 146115 |
| 8 | 1855570 |
| 9 | 26097835 |
| 10 | 402215465 |
| 11 | 6734414075 |
| 12 | 121629173423 |
| 13 | 2355470737637 |
| 14 | 48664218965021 |
| 15 | 1067895971109199 |
| 16 | 24795678053493443 |
| 17 | 607144847919796830 |
| 18 | 15630954703539323090 |
| 19 | 421990078975569031642 |
| 20 | 11918095123121138408128 |
[1,1,4,22,154,1304,12915,146115,1855570,26097835, 402215465,6734414075,121629173423,2355470737637, 48664218965021,1067895971109199,24795678053493443, 607144847919796830,15630954703539323090, 421990078975569031642,11918095123121138408128] |