concept of test function in calculus

Hi everyone,

I was reading about the dirac delta function which is defined as an im
pulse which has infinite magnitude at x = 0 and is 0 everywhere else.

However, when I look at the mathematical equation describing it on
wikipedia and other books, it is as follows:

Integral (-inf, +inf) f(x) del(x) dx = f(0)

I thought Integral (-inf, +inf) del(x) dx = 1 should be sufficient.

What is this f(x) function and why is it used? Wikipedia says it is
some sort of a "test function" but I am not sure as to what it means!

I would be grateful if someone could explain this to me.

Thanks,

Luca

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