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### Functions in polynomials rings

I want to define a function in a polynomial ring in several variables.

R.<x1,x3,x5>=PolynomialRing(QQ)


I am trying to define a function that takes $(i,j)$ to $x_i^j$.

I tried

def f(i,j):
return xi^j


This does not work. I tried replacing xi with x[i], that doesn't work. Can someone please tell me what I am doing wrong and how to fix it? If instead of taking 3 variables I take only 1 variable then the method works.