how to denest the sum symbol in sum of sympy.Sum

[chaowen guo]

Hi:

I want to use power series to solve second order differential equation. Albeit sympy.ode already has 2nd_power_series_ordinary and 2nd_power_series_regular to do the job, it does not provide the recurrence relation of coefficient. What I actually need is the recurrence relation of coefficient. So I follow the standard procedure of power series



and apply it to sturm liouville form of legendre equation. The next job is to get the coefficient of x**i, I can not directly use legendre.coeffs(x,i), so I want to extract the sum symbol outside just like that in the last markdown and then use args to extract the expression inside sum and I can do more operation. The problem is how to extract the sum symbol outside just like that in the last markdown. sympy.extend and sympy.factor do not do the job. Which function in sympy actually the job?