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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 106 |
| 5 | 750 |
| 6 | 6188 |
| 7 | 58184 |
| 8 | 613156 |
| 9 | 7149780 |
| 10 | 91319712 |
| 11 | 1267089912 |
| 12 | 18969355656 |
| 13 | 304646227704 |
| 14 | 5222700792528 |
| 15 | 95169251327040 |
| 16 | 1836450816902928 |
| 17 | 37403582826055824 |
| 18 | 801728489886598848 |
| 19 | 18037821249349491360 |
| 20 | 424970923585819603872 |
| 21 | 10462258547232790348512 |
[1,1,4,18,106,750,6188,58184,613156,7149780, 91319712,1267089912,18969355656,304646227704, 5222700792528,95169251327040,1836450816902928, 37403582826055824,801728489886598848, 18037821249349491360,424970923585819603872, 10462258547232790348512] |