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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 36 |
| 3 | 296 |
| 4 | 2732 |
| 5 | 28980 |
| 6 | 329996 |
| 7 | 3872908 |
| 8 | 46575260 |
| 9 | 573472248 |
| 10 | 7197096168 |
| 11 | 91640952360 |
| 12 | 1180636398320 |
| 13 | 15364364313588 |
| 14 | 201691201775092 |
| 15 | 2667523242203932 |
| 16 | 35510152549696208 |
| 17 | 475424653523498396 |
| 18 | 6397601663340197268 |
| 19 | 86481499341290372804 |
| 20 | 1173813146742741571560 |
[1,4,36,296,2732,28980,329996,3872908,46575260, 573472248,7197096168,91640952360,1180636398320, 15364364313588,201691201775092,2667523242203932, 35510152549696208,475424653523498396, 6397601663340197268,86481499341290372804, 1173813146742741571560] |