In differential calculus, an equation in which the rate of change of a set of variables is a function of the current values of the variables. Solutions of differential equations are time courses of all variables. These time courses are uniquely determined given the initial values of the variables. The order of a differential equation is the order of its highest derivative, and its degree the highest power of the highest derivative (exponent). A derivative (or differential coefficient), which is equal to the gradient of the curve (i.e., the gradient of the tangent to the curve at point x), is the outcome of the differentiation of a mathematical function such that the first derivative is the rate of change (d) of the value of the function with respect to the independent variable. Thus, for y = xⁿ, where x is the independent variable, the derivative is dy/dx = nxⁿ⁻¹. The mathematical theory of dynamical systems deals with how properties of the solutions (e.g., stability) depend on properties of the differential equation. There are two main types of differential equations: ordinary differential equation (an equation describing a prescribed relationship between a set of unknowns, and which has only one independent variable) and a partial differential equation (an equation involving partial derivatives of unknown function).