| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 22 |
| 3 | 218 |
| 4 | 3048 |
| 5 | 54852 |
| 6 | 1206696 |
| 7 | 31373856 |
| 8 | 941214240 |
| 9 | 32001274080 |
| 10 | 1216048334400 |
| 11 | 51074029319040 |
| 12 | 2349405341418240 |
| 13 | 117470266991078400 |
| 14 | 6343394416560230400 |
| 15 | 367916876148039321600 |
| 16 | 22810846321004081356800 |
| 17 | 1505515857183654020812800 |
| 18 | 105386110002813935877120000 |
[4,22,218,3048,54852,1206696,31373856,941214240, 32001274080,1216048334400,51074029319040, 2349405341418240,117470266991078400, 6343394416560230400,367916876148039321600, 22810846321004081356800,1505515857183654020812800, 105386110002813935877120000] |