Computing Riemann-Roch Spaces in Algebraic Function Fields and Related Topics

We develop a simple and efficient algorithm to compute Riemann---Roch spaces of divisors in general algebraic function fields which does not use the Brill-Noether method of adjoints or any series expansions. The basic idea also leads to an elementary proof of the Riemann-Roch theorem. We describe the connection to the geometry of numbers of algebraic function fields and develop a notion and algorithm for divisor reduction. An important application is to compute in the divisor class group of an algebraic function field.

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