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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 126 |
| 5 | 962 |
| 6 | 8646 |
| 7 | 89546 |
| 8 | 1051350 |
| 9 | 13811642 |
| 10 | 200866326 |
| 11 | 3205348346 |
| 12 | 55704133590 |
| 13 | 1047489675962 |
| 14 | 21195507649686 |
| 15 | 459258707587706 |
| 16 | 10610386574074710 |
| 17 | 260385846630175802 |
| 18 | 6764666952807962646 |
[1,1,4,20,126,962,8646,89546,1051350,13811642, 200866326,3205348346,55704133590,1047489675962, 21195507649686,459258707587706,10610386574074710, 260385846630175802,6764666952807962646] |