The rational form of a matrix is a similar matrix composed of submatrices (“blocks”) placed on the main diagonal. Each block is a companion matrix. Associated with each companion matrix is a polynomial. In rational form, the polynomial of one block will divide the polynomial of the next block (and thus, the polynomials of all subsequent blocks).
Rational form, also known as Frobenius form, is a canonical form. In other words, two matrices are similar if and only if their rational canonical forms are equal. The algorithm used does not provide the similarity transformation matrix (also known as the change-of-basis matrix).
Companion matrices may be written in one of four styles, and any such style may be selected with the
format keyword. See the companion matrix constructor,
sage.matrix.constructor.companion_matrix(), for more information about
companion matrices.
If the ‘invariants’ value is used for the
format keyword, then the return value is a list of lists, where each list is the coefficients of the polynomial associated with one of the companion matrices on the diagonal. These coefficients include the leading one of the monic polynomial and are ready to be coerced into any polynomial ring over the same field (see examples of this below). This return value is intended to be the most compact representation and the easiest to use for testing equality of rational forms.
Because the minimal and characteristic polynomials of a companion matrix are the associated polynomial, it is easy to see that the product of the polynomials of the blocks will be the characteristic polynomial and the final polynomial will be the minimal polynomial of the entire matrix.
