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On reducing the Heun equation to the hypergeometric equation. (English) Zbl 1085.34035
Summary: The reductions of the Heun equation to the hypergeometric equation by polynomial transformations of its independent variable are enumerated and classified. Heun-to-hypergeometric reductions are similar to classical hypergeometric identities, but the conditions for the existence of a reduction involve features of the Heun equation that the hypergeometric equation does not possess; namely, its cross-ratio and accessory parameters. The reductions include quadratic and cubic transformations, which may be performed only if the singular points of the Heun equation form a harmonic or an equianharmonic quadruple, respectively; and several higher-degree transformations. This result corrects and extends a theorem in a previous paper, which found only the quadratic transformations; see K. Kuiken [SIAM J. Math. Anal. 10, 655–657 (1979; Zbl 0415.34038)].
34C20Transformation and reduction of ODE and systems, normal forms