Maurice Janet's algorithms on systems of linear partial differential equations

TL;DR

This paper explores Maurice Janet's pioneering formal methods from 1913-1930 for analyzing the solvability and compatibility of linear PDE systems using algorithmic and algebraic techniques involving monomials and multiplicative sets.

Contribution

It highlights Janet's original algorithmic approach to PDE compatibility and initial conditions, laying groundwork for later algebraic completion methods.

Findings

Janet introduced an algorithmic method for compatibility conditions.

He developed a division operation on monomials for PDE analysis.

Janet's completion procedure was the first in polynomial algebra.

Abstract

This article presents the emergence of formal methods in theory of partial differential equations (PDE) in the french school of mathematics through Janet's work in the period 1913-1930. In his thesis and in a series of articles published during this period, M. Janet introduced an original formal approach to deal with the solvability of the problem of initial conditions for finite linear PDE systems. His constructions implicitly used an interpretation of a monomial PDE system as a generating family of a multiplicative set of monomials. He introduced an algorithmic method on multiplicative sets to compute compatibility conditions, and to study the problem of the existence and the unicity of a solution to a linear PDE system with given initial conditions. The compatibility conditions are formulated using a refinement of the division operation on monomials defined with respect to a partition of the set of variables into multiplicative and non-multiplicative variables. M. Janet was a pioneer in the development of these algorithmic methods, and the completion procedure that he introduced on polynomials was the first one in a long and rich series of works on completion methods which appeared independently throughout the 20th century in various algebraic contexts.