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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 27 |
| 3 | 224 |
| 4 | 2070 |
| 5 | 20444 |
| 6 | 211239 |
| 7 | 2255200 |
| 8 | 24680862 |
| 9 | 275408456 |
| 10 | 3121711758 |
| 11 | 35842765872 |
| 12 | 415999482912 |
| 13 | 4872611960268 |
| 14 | 57524376899871 |
| 15 | 683778868508352 |
| 16 | 8176889078761590 |
| 17 | 98303183244150968 |
| 18 | 1187413394405415498 |
| 19 | 14403838768083054208 |
| 20 | 175394125615368312900 |
[1,4,27,224,2070,20444,211239,2255200,24680862, 275408456,3121711758,35842765872,415999482912, 4872611960268,57524376899871,683778868508352, 8176889078761590,98303183244150968, 1187413394405415498,14403838768083054208, 175394125615368312900] |