I'm a little confused about the gaussian filter function which can be found on several sources.

What's confusing to me is that I've found at least 3 different equations for the same thing...

I've marked the differences in red color.

Wikipedia lists it as: (http://en.wikipedia.org/wiki/Gaussian_filter)

g(x) = (1 / (sqrt(2 * PI) * rho)) * exp(-(x*x) / (2 * (rho * rho)))

While the one found in Real-Time-Rendering, Third Edition defines it as:

g(x) = (1 / (sqrt(2 * PI) * rho * rho)) * exp(-(x*x) / (2 * (rho * rho)))

And finally the code that I've got from one of MJP's samples:

g(x) = (1 / (sqrt(2 * PI * rho * rho))) * exp(-(x*x) / (2 * (rho * rho)))

What am I missing here ?