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| n | a(n) |
|---|---|
| 1 | 2 |
| 2 | 2 |
| 3 | 42 |
| 4 | 228 |
| 5 | 1644 |
| 6 | 10368 |
| 7 | 65182 |
| 8 | 414414 |
| 9 | 2603670 |
| 10 | 16540506 |
| 11 | 104489208 |
| 12 | 662121980 |
| 13 | 4188014160 |
| 14 | 26506383680 |
| 15 | 167699955140 |
| 16 | 1061128778004 |
| 17 | 6714156638826 |
| 18 | 42483540261942 |
| 19 | 268814679252084 |
| 20 | 1700917160828256 |
| 21 | 10762539724166324 |
[2,2,42,228,1644,10368,65182,414414,2603670, 16540506,104489208,662121980,4188014160, 26506383680,167699955140,1061128778004, 6714156638826,42483540261942,268814679252084, 1700917160828256,10762539724166324] |