Let F(θ) be a separable extension of degree n of a field F.Let Δ and D be integral domains with quotient fields F(θ) and F respectively.Assume that Δ ᴝ D.A mapping φ of Δ into the n x n D matrices is called a Δ/D rep if (i) it is a ring isomorphism and (ii) it maps d onto dIn whenever d ϵ D.If the matrices are also symmetric, φ is a Δ/D symrep.
Every Δ/D rep can be extended uniquely to an F(θ)/F rep.This extension is completely determined by the image of θ.Two Δ/D reps are called equivalent if the images of θ differ by a D unimodular similarity.There is a one-to-one correspondence between classes of Δ/D reps and classes of Δ ideals having an n element basis over D.
The condition that a given Δ/D rep class contain a Δ/D symrep can be phrased in various ways.Using these formulations it is possible to (i) bound the number of symreps in a given class, (ii) count the number of symreps if F is finite, (iii) establish the existence of an F(θ)/F symrep when n is odd, F is an algebraic number field, and F(θ) is totally real if F is formally real (for n = 3 see Sapiro, “Characteristic polynomials of symmetric matrices” Sibirsk. Mat. Ž. 3 (1962) pp. 280-291), and (iv) study the case D = Z, the integers (see Taussky, “On matrix classes corresponding to an ideal and its inverse” Illinois J. Math. 1 (1957) pp. 108-113 and Faddeev, “On the characteristic equations of rational symmetric matrices” Dokl. Akad. Nauk SSSR 58 (1947) pp. 753-754).
The case D = Z and n = 2 is studied in detail.Let Δ’ be an integral domain also having quotient field F(θ) and such that Δ’ ᴝ Δ.Let φ be a Δ/Z symrep.A method is given for finding a Δ’/Z symrep ʘ such that the Δ’ ideal class corresponding to the class of ʘ is an extension to Δ’ of the Δ ideal class corresponding to the class of φ.The problem of finding all Δ/Z symreps equivalent to a given one is studied.
