undefined expression

For what real values of a is the expression (a + 3)/(a^3 - a) undefined? List your answers in increasing order separated by commas.

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(a + 3)/(a^3 - a) is undefined if the term a^3 - a is zero.

$a^3-a=a(a^2-1)=0\\ a=0,\ a^2-1=0\\ a\in\{0,1,-1\}$

$(a + 3)/(a^3 - a) \ is\ undefined\ | a\in\{-1,0,1\}$

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