In mathematics, in the theory of ordinary differential equations in the complex plane
are classified into ordinary points, at which the equation's coefficients are analytic functions, and singular points, at which some coefficient has a singularity.
Then amongst singular points, an important distinction is made between a regular singular point, where the growth of solutions is bounded (in any small sector) by an algebraic function, and an irregular singular point, where the full solution set requires functions with higher growth rates.
This distinction occurs, for example, between the hypergeometric equation, with three regular singular points, and the Bessel equation which is in a sense a limiting case, but where the analytic properties are substantially different.
More precisely, consider an ordinary linear differential equation of n-th order
with pi(z) meromorphic functions.
The equation should be studied on the Riemann sphere to include the point at infinity as a possible singular point.
A Möbius transformation may be applied to move ∞ into the finite part of the complex plane if required, see example on Bessel differential equation below.
Then the Frobenius method based on the indicial equation may be applied to find possible solutions that are power series times complex powers (z − a)r near any given a in the complex plane where r need not be an integer; this function may exist, therefore, only thanks to a branch cut extending out from a, or on a Riemann surface of some punctured disc around a.
This presents no difficulty for a an ordinary point (Lazarus Fuchs 1866).
When a is a regular singular point, which by definition means that
has a pole of order at most i at a, the Frobenius method also can be made to work and provide n independent solutions near a.
In that case the monodromy group relating solutions by analytic continuation has less to say in general, and the solutions are harder to study, except in terms of their asymptotic expansions.
The regularity condition is a kind of Newton polygon condition, in the sense that the allowed poles are in a region, when plotted against i, bounded by a line at 45° to the axes.
An ordinary differential equation whose only singular points, including the point at infinity, are regular singular points is called a Fuchsian ordinary differential equation.
One distinguishes the following cases: We can check whether there is an irregular singular point at infinity by using the substitution
are quotients of polynomials, then there will be an irregular singular point at infinite x unless the polynomial in the denominator of
Listed below are several examples from ordinary differential equations from mathematical physics that have singular points and known solutions.
This is an ordinary differential equation of second order.
It is found in the solution to Laplace's equation in cylindrical coordinates:
for an arbitrary real or complex number α (the order of the Bessel function).
The most common and important special case is where α is an integer n. Dividing this equation by x2 gives:
This is an ordinary differential equation of second order.
It is found in the solution of Laplace's equation in spherical coordinates:
This differential equation has regular singular points at ±1 and ∞.
One encounters this ordinary second order differential equation in solving the one-dimensional time independent Schrödinger equation
This leads to the following ordinary second order differential equation:
This differential equation has an irregular singularity at ∞.
{\displaystyle {\frac {d^{2}f}{dz^{2}}}+{\frac {c-(a+b+1)z}{z(1-z)}}{\frac {df}{dz}}-{\frac {ab}{z(1-z)}}f=0.}
This differential equation has regular singular points at 0, 1 and ∞.