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### Abstract

We verify two algorithms for which modular arithmetic plays an
essential role: Storjohann's variant of the LLL lattice basis
reduction algorithm and Kopparty's algorithm for computing the
Hermite normal form of a matrix. To do this, we also formalize some
facts about the modulo operation with symmetric range. Our
implementations are based on the original papers, but are otherwise
efficient. For basis reduction we formalize two versions: one that
includes all of the optimizations/heuristics from Storjohann's
paper, and one excluding a heuristic that we observed to often
decrease efficiency. We also provide a fast, self-contained certifier
for basis reduction, based on the efficient Hermite normal form
algorithm.