Jacobi's Bound. Jacobi's results translated in K{Ö}nig's, Egerv{á}ry's and Ritt's mathematical languages

Abstract

Jacobi's results on the computation of the order and of the normal forms of a differential system are translated in the formalism of differential algebra. In the quasi-regular case, we give complete proofs according to Jacobi's arguments. The main result is {\it Jacobi's bound}, still conjectural in the general case: the order of a differential system $P_{1}, \ldots, P_{n}$ is not greater than the maximum $\cal O$ of the sums $\sum_{i=1}^{n} a_{i,σ(i)}$, for all permutations $σ$ of the indices, where $a_{i,j}:={\rm ord}_{x_{j}}P_{i}$, \emph{viz.}\ the \emph{tropical determinant of the matrix $(a_{i,j})$}. The order is precisely equal to $\cal O$ iff Jacobi's \emph{truncated determinant} does not vanish. Jacobi also gave a polynomial time algorithm to compute $\cal O$, similar to Kuhn's "Hungarian method" and some variants of shortest path algorithms, related to the computation of integers $\ell_{i}$ such that a normal form may be obtained, in the generic case, by differentiating $\ell_{i}$ times equation $P_{i}$. Fundamental results about changes of orderings and the various normal forms a system may have, including differential resolvents, are also provided.