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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 12 |
| 3 | 60 |
| 4 | 364 |
| 5 | 2448 |
| 6 | 17556 |
| 7 | 131560 |
| 8 | 1017900 |
| 9 | 8069424 |
| 10 | 65204656 |
| 11 | 535070172 |
| 12 | 4446927732 |
| 13 | 37353738800 |
| 14 | 316621743480 |
| 15 | 2704784196240 |
| 16 | 23263187479980 |
| 17 | 201275443944432 |
| 18 | 1750651680235920 |
| 19 | 15298438066553776 |
| 20 | 134252511729576240 |
| 21 | 1182622941581590080 |
[4,12,60,364,2448,17556,131560,1017900,8069424, 65204656,535070172,4446927732,37353738800, 316621743480,2704784196240,23263187479980, 201275443944432,1750651680235920, 15298438066553776,134252511729576240, 1182622941581590080] |