Thx~~
But, this term comes from the Mathieu’s equation as below
\frac{d^{2}f\left(x\right)}{dx^{2}}+\left(\epsilon-\cos\frac{d}{dx}\right)f\left(x\right)=0
If I use series, this equation would be so terrible
I have a queation as below
[cos(d/dx)]f(x)
How to solve it? I just konw that the angle is an operator, but the function of x is out of the cosine function.