In mathematics, the Fuchs relation is a relation between the starting exponents of formal series solutions of certain linear differential equations, so called Fuchsian equations. It is named after Lazarus Immanuel Fuchs.

Definition Fuchsian equation

A linear differential equation in which every singular point, including the point at infinity, is a regular singularity is called Fuchsian equation or equation of Fuchsian type.[1] For Fuchsian equations a formal fundamental system exists at any point, due to the Fuchsian theory.

Coefficients of a Fuchsian equation

Let be the regular singularities in the finite part of the complex plane of the linear differential equation

with meromorphic functions . For linear differential equations the singularities are exactly the singular points of the coefficients. is a Fuchsian equation if and only if the coefficients are rational functions of the form

with the polynomial and certain polynomials for , such that .[2] This means the coefficient has poles of order at most , for .

Fuchs relation

Let be a Fuchsian equation of order with the singularities and the point at infinity. Let be the roots of the indicial polynomial relative to , for . Let be the roots of the indicial polynomial relative to , which is given by the indicial polynomial of transformed by at . Then the so called Fuchs relation holds:

.[3]

The Fuchs relation can be rewritten as infinite sum. Let denote the indicial polynomial relative to of the Fuchsian equation . Define as

where gives the trace of a polynomial , i. e., denotes the sum of a polynomial's roots counted with multiplicity.

This means that for any ordinary point , due to the fact that the indicial polynomial relative to any ordinary point is . The transformation , that is used to obtain the indicial equation relative to , motivates the changed sign in the definition of for . The rewritten Fuchs relation is:

[4]

References

  • Ince, Edward Lindsay (1956). Ordinary Differential Equations. New York, USA: Dover Publications. ISBN 9780486158211.
  • Tenenbaum, Morris; Pollard, Harry (1963). Ordinary Differential Equations. New York, USA: Dover Publications. pp. Lecture 40. ISBN 9780486649405.
  • Horn, Jakob (1905). Gewöhnliche Differentialgleichungen beliebiger Ordnung. Leipzig, Germany: G. J. Göschensche Verlagshandlung.
  • Schlesinger, Ludwig (1897). Handbuch der Theorie der linearen Differentialgleichungen (2. Band, 1. Teil). Leipzig, Germany: B. G.Teubner. pp. 241 ff.
  1. Ince, Edward Lindsay (1956). Ordinary Differential Equations. New York, USA: Dover Publications. p. 370. ISBN 9780486158211.
  2. Horn, Jakob (1905). Gewöhnliche Differentialgleichungen beliebiger Ordnung. Leipzig, Germany: G. J. Göschensche Verlagshandlung. p. 169.
  3. Ince, Edward Lindsay (1956). Ordinary Differential Equations. New York, USA: Dover Publications. p. 371. ISBN 9780486158211.
  4. Landl, Elisabeth (2018). The Fuchs Relation (Bachelor Thesis). Linz, Austria. chapter 3.
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