In transcendental number theory and Diophantine approximation, Siegel's lemma refers to bounds on the solutions of linear equations obtained by the construction of auxiliary functions. The existence of these polynomials was proven by Axel Thue; Thue's proof used Dirichlet's box principle. Carl Ludwig Siegel published his lemma in 1929. It is a pure existence theorem for a system of linear equations.
Siegel's lemma has been refined in recent years to produce sharper bounds on the estimates given by the lemma.
Statement
Suppose we are given a system of M linear equations in N unknowns such that N > M, say
where the coefficients are rational integers, not all 0, and bounded by B. The system then has a solution
with the Xs all rational integers, not all 0, and bounded by
Bombieri & Vaaler (1983) gave the following sharper bound for the X's:
where D is the greatest common divisor of the M by M minors of the matrix A, and AT is its transpose. Their proof involved replacing the Dirichlet box principle by techniques from the geometry of numbers.