Fuchs' theorem

In mathematics, Fuchs' theorem, named after Lazarus Fuchs, states that a second-order differential equation of the form $$y^{″}+p(x)y^{\prime }+q(x)y=g(x)$$ has a solution expressible by a generalised Frobenius series when $p(x)$, $q(x)$ and $g(x)$ are analytic at $x=a$ or $a$ is a regular singular point. That is, any solution to this second-order differential equation can be written as $$y=\sum _{n=0}^{\mathrm{\infty }}{a}_n(x-a{)}^{n+s},a_0\ne 0$$ for some positive real s, or $$y=y_0\ln (x-a)+\sum _{n=0}^{\mathrm{\infty }}{b}_n(x-a{)}^{n+r},b_0\ne 0$$ for some positive real r, where y₀ is a solution of the first kind.

Its radius of convergence is at least as large as the minimum of the radii of convergence of $p(x)$, $q(x)$ and $g(x)$.

See also

• Frobenius method

References

• Asmar, Nakhlé H. (2005), Partial differential equations with Fourier series and boundary value problems, Upper Saddle River, NJ: Pearson Prentice Hall, ISBN 0-13-148096-0 .
• Butkov, Eugene (1995), Mathematical Physics, Reading, MA: Addison-Wesley, ISBN 0-201-00727-4 .