The equation for this operator requires the inverse of the metric tensor g and its determinant.

(this equation requires a consistent system of measurement)

The equation requires an iterative solution to extract the pressure drop, as it is present on both sides of the equation.

(Most notably, Dirac predicted the existence of antimatter in 1928 because his just discovered relativistic electron equation required it.)

The region close to the pivot is singular, since is close to zero and the equations of motion require dividing by .

The resulting ordinary differential equations are stiff and require therefore special attention.

The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.

To solve a linear equation with two variables (unknowns), requires two related equations.

Schroedinger's equation requires differential equations and probability functions to describe "quantum mechanics."

This equation is very simple to implement and requires only that the 3D FT of the basis functions be computed, preferably in closed form.