For clarifications see http://martin.ankerl.com/2007/02/11/optimized-exponential-functions-for-java/
For clarifications see https://www.reddit.com/r/gamedev/comments/n7na0/fast_approximation_to_mathpow/ and https://martin.ankerl.com/2007/10/04/optimized-pow-approximation-for-java-and-c-c/
Not very good pow and exponent approximations consider org.apache.commons.math{3|4}.util.FastMath usage in cases where these functions can't be applied.
ApproximationsSpec demonstrates max deviation on SaturationAdjust and SigmoidalContrast domains