=是赋值,a[2] = 0 结果为0
相当于求：
RSolve[{a[n + 3] == 2/3*a[n] + a[n + 1]/3, a[1]==2/3,0,2/9}, a[n], n]
C[1],C[2]是任意常数.
其实,应该：
RSolve[{a[n + 3] == 2/3*a[n] + a[n + 1]/3, a[1] == 2/3, a[2] == 0,
a[3] == 2/9}, a[n], n]
a[n] -> (15 (I + Sqrt[15]) + (45 I -
11 Sqrt[15]) (-(1/6) I (-3 I + Sqrt[15]))^n -
4 (-(1/2) + 1/2 I Sqrt[5/3])^n (15 I + Sqrt[15]))/(
60 (I + Sqrt[15]))
a[n] ->1/4+ ( (45 I -
11 Sqrt[15]) (-(1/6) I (-3 I + Sqrt[15]))^n -
4 (-(1/2) + 1/2 I Sqrt[5/3])^n (15 I + Sqrt[15]))/(
60 (I + Sqrt[15]))