Sequence of function Gn(x)

Good Afternoon from Indonesia,

I really need a help regarding how to write a sequence of function like this.

G(t) is the probability of "n-th" failed component occurs at the "Un" specific value of time. (n = 1,2,3 ....., k)

I wrote the code like this:

syms n a
G(t,n) =1 - symsum(exp(-L_c(t)) * (L_c(t))^a/factorial(a), a, 0, n-1);

I still hesitate that my code whether is right or wrong, because it still generate an unsatisfactorily result. Would anyone here come to evaluate the code written by me?

Thank you