What is the longest math equation?

well, by definition, a math equation is something along the lines of y=x, where either y or x can be anything, including functions or non functions. so really you could just make up anything as an equation, which means there really isn't a math equation that is the longest. for example, i could say y=x^3 +2x^2+4x+2.5x^(1/2)+cuberoot(34x^2)... yadda yadda yadda, and it would be an equation. basically there is an infinite number of terms you could have in an equation.

a more suitable question would be to ask,"what is the longest applicable math equation?", which i think you probably meant in the first place. just be weary about how you use the Word "equation."

as for the longest applicable math equation, look at things like Nordstrand's Weird Surface, or in general, Integral Transforms are quite lengthy. applicable equations don't necessarily have to be very long to be complicated.