What does it mean to have an integral with a lower bound of infinity?

[dmatador]

I am working on a proof in which I have an integral with bounds negative infinity to zero, with an even function, i.e., f(y) = f(-y). I took the limit to infinity rather than negative infinity since y is negative (which is OK I think) but now I have an integral that goes from infinity to 0. What can I make of this? I remember something about this from calculus, but nothing in particular.

 

[Mark44]

I am working on a proof in which I have an integral with bounds negative infinity to zero, with an even function, i.e., f(y) = f(-y). I took the limit to infinity rather than negative infinity since y is negative (which is OK I think) but now I have an integral that goes from infinity to 0. What can I make of this? I remember something about this from calculus, but nothing in particular.

\int_a^b f(x) dx = -\int_b^a f(x) dx

 

[dmatador]

So this holds even when a or b is infinity? If so, thanks for the help. My proof is done.