Does anyone know a mathematical approximation for how Photoshop interprets the "gamma" slider setting?

I believe it's an exponential function, similar to the normal gamma curve so many of us know and love, with a modified slope for the dark end of the scale. I'm after an explicit equation.

TIA
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Mike Russell
http://www.curvemeister.com
http://www.zocalo.net/~mgr
http://geigy.2y.net