Okay, I have made up this rule. I would love to call it mine, but I am sure some one is bound to have invented it by now... f(x) find a such that f '(a)=0 now find the first nth derivative such that f^n(a) !=0 Usually n=2. 1. If n E odd - Inflexion, If n E even, keep going... 2. If f^n(a)>0 - Minima, If f^n(a)<0 - Maxima. Well that is the jist of it... Is it valid?