I have two functions which are eigenvalues: $$ E_+= a*x-\sqrt{(b-x^2-y^2)^2+d*(x^2+y^2)}$$ ...and... $$ E_-= a*x+\sqrt{(b-x^2-y^2)^2+d*(x^2+y^2)}. $$ How do I define a region $\left|E_{\pm}\right|<E_f$ where $E_f$ is a constant and $x,y$ are the variables for the region?
Context: this region is the region inside Fermi surface of a solid state system which shall be my region for integrating a big function using NIntegrate
.