a(4)=7:D^2[(1+x)*exp(x)]=exp(2*x)*(2*x^2+8*x+7)。在x=0时计算得出a(4)=7。用字母a、b、c表示节点的颜色,。……3个节点上的7个可能的树,其节点的等级为k,以k+1颜色表示,如下所示:
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…1a。…1b。…1a。…1b。……….1a。……1b。…….1c。...
...|.....|.....|.....|......../.\....../..\....../..\...
…2a。…2b。…2b。…2a。.....2...3....2....3....2....3..
...|.....|.....|.....|..................................
...3.....3.....3.....3..................................
G.f.=x+x ^2+2*x ^3+7*x ^4+36*x ^5+245*x ^6+2076*x ^7+21059*x ^8+。..