This problem is in the context of discrete systems and signals. My goal is to find an equation for y(n) without the summation. where is the unit step function, k is from -inf to inf, and n=0,1,2,3... So I need to eliminate the unit step functions from the equation by changing the limits of summation: if then . And so it's eliminated from my summation above. if then . And it's also eliminated. So my limits of summation are and . But the limits don't define a finite range! Can anyone help?