calling Legendre function

Dear root users,
I could not figure out how to call Legendre functions in a code, without using TF1 object. I don't want to plot it or use it in a fit, I just want to use it in a calculation such that:

S(x) = A1*Legendre(1,x) + A2*Legendre(2,x) + ...

I appreciate your guidance.
Thank you,

• Nevzat Guler (757) 553-8318