III. A memoir on the theory of matrices

Abstract

The term matrix might be used in a more general sense, but in the present memoir I consider only square and rectangular matrices, and the term matrix used without qualification is to be understood as meaning a square matrix ; in this restricted sense, a set of quan­tities arranged in the form of a square, e. g. ( a, b, c ) | a´, b´, c´ a´´, b´´, c´´ | is said to be a matrix. The notation of such a matrix arises naturally from an abbreviated notation for a set of linear equations, viz. the equations X= ax + by + cz Y= a´x + b´y + c´z Z= a´´x + b´´y + c´´z may be more simply represented by (X, Y, Z)=( a, b, c )( x,y,z ) | a´, b´, c´ a´´, b´´, c´´ | and the consideration of such a system of equations leads to most of the fundamental notions in the theory of matrices. It will be seen that matrices (attending only to those of the same degree) com­port themselves as single quantities ; they may be added, multiplied, or compounded together, &c.: the law of the addition of matrices is precisely similar to that for the addition of ordinary algebraical quan­tities ; as regards their multiplication (or composition), there is the peculiarity that matrices are not in general convertible; it is never­theless possibleato form the powers (positive or negative, integral or fractional) of a matrix, and thence to arrive at the notion of a rational and integral function, or generally of any algebraical func­tion of a matrix. I obtain the remarkable theorem that any matrix whatever satisfies an algebraical equation of its own order, the coeffi­cient of the highest power being unity, and those of the other powers functions of the terms of the matrix, the last coefficient being in fact the determinant. The rule for the formation of this equation may be stated in the following condensed form, which will be intelligible after a perusal of the memoir, viz. the determinant, formed out of the matrix diminished by the matrix considered as a single quantity involving the matrix unity, will be equal to zero. The theorem shows that every rational and integral function (or indeed every rational function) of a matrix may be considered as a rational and integral function, the degree of which is at most equal to that of the matrix, less unity ; it even shows that in a sense, the same is true with respect to any algebraical function whatever of a matrix. One of the applications of the theorem is the finding of the general expression of the matrices which are convertible with a given matrix. The theory of rectangular matrices appears much less important than that of square matrices, and I have not entered into it further than by showing how some of the notions applicable to these may be extended to rectangular matrices.