Data type problem

I have the following code which calculates Legende's polynomials.

Code:

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((2*n-1)*x*leg(n-1, x) - (n-1)*leg(n-2, x))/n

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The problem is that the function doesn't work when I input negative parameters. Is there something I'm missing?

EDIT: Okay, I'm an idiot. n is not supposed to be a negative.