Abstract
As its title suggests, this chapter is devoted to tying up several historical loose ends related to the work of Frobenius featured in the previous chapters. The first section focuses on work done by Frobenius in response to the discovery of a gap in Weierstrass’ theory of elementary divisors as it applied to families of quadratic forms. Frobenius gave two solutions to the problem of filling the gap. The first drew upon the results and analogical reasoning used in his arithmetic theory of bilinear forms and its application to elementary divisor theory (Chapter 8).
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Notes
- 1.
This fact and the other information preliminary to Frobenius’ discovery of a proof is drawn from Frobenius’ account in 1894 [203, pp. 577–578].
- 2.
In the case of algebraic integers, some results require excluding p a prime divisor of 2 [203, p. 584] in the definition of “regular with respect to p.”
- 3.
On Kronecker’s work with fields, see Purkert’s study [491].
- 4.
Frobenius’ theorem actually specifies that \(\deg \chi = m - 1\), where m is the degree of the minimal polynomial of U. This additional information can be inferred from Frobenius’ proof; see below.
- 5.
This is indeed correct, as Frobenius showed in his 1878 paper [181, p. 355], assuming that when \(f(t) = p(t)/q(t)\), detq(A) ≠ 0, so that [q(A)] − 1 exists.
- 6.
If L 2 = M, then \({L}^{4} = {M}^{2} = 0\). This means that the characteristic roots of L must all be 0 and so (since L is 2 ×2) \(\varphi (t) = {t}^{2}\) is the characteristic polynomial of L. The Cayley–Hamilton theorem then implies \({L}^{2} = 0\not =M\), and so \(\sqrt{M}\) does not exist.
- 7.
- 8.
I refer to Lagrange’s attempt to extend his elegant generic solution to \(\ddot{y} + Ay = 0\), y(0) = y 0, A n ×n, to the case in which \(f(\rho ) =\det {(\rho }^{2} + A)\) has one root of multiplicity two. See Section 4.2.1.
- 9.
See in this connection Frobenius’ use of power and Laurent series in his proof of his minimal polynomial theorem (Theorem 7.2) and in his proof that a real orthogonal matrix can be diagonalized (Theorem 7.15).
- 10.
Frobenius’ proof [208, pp. 697ff.] is expressed somewhat more generally than expounded here so as to allow a brief discussion of the problematic case det U = 0 as well as other functions of a matrix.
- 11.
Frobenius proved that if S is a matrix with the property that the sequence S j, j = 0, 1, 2, …, has only a finite number of distinct terms, then all characteristic roots of S are either 0 or roots of unity, and the elementary divisors corresponding to the roots of unity are all linear [181, Satz VI, p. 357].
- 12.
See in this connection [270, p. 107n.15].
- 13.
Regarding Muth’s life and work, see [463].
- 14.
In the preface, Muth wrote that he had been encouraged by the fact that “From the outset my undertaking was of special interest to several outstanding experts in the theory of elementary divisors,” namely Frobenius, S. Gundelfinger, and K. Hensel (Kronecker’s former student) [450, p. iv].
- 15.
- 16.
Kronecker ignored the work of Smith, even though Frobenius referenced it throughout his paper.
- 17.
For a contemporary rendition, see, e.g., [141, pp. 459ff.].
- 18.
Weber’s Lehrbuch der Algebra in its various editions from 1895 onward did not treat elementary divisor theory.
- 19.
Bôcher also mentioned (1) λ-polynomials with integer coefficients and (2) polynomials in several variables; but it is unclear what he thought could be proved in these two cases, neither of which involves a principal ideal domain.
- 20.
Let \(A ={\biggl ( \begin{array}{ccc} \rho _{1}\rho _{2}\! & \!0 \\ 0\! & \!1 \\ \end{array} \biggr )}\) and \(B ={\biggl ( \begin{array}{ccc} \rho _{1}\! & \!0 \\ 0\! & \!\rho _{2} \\ \end{array} \biggr )}\). Then A and B have the same invariant factors but B = P A Q is impossible, because it implies detPdetQ = 1 as well as that detP depends on the ρ i and vanishes for \(\rho _{1} =\rho _{2} = 0\) [425, p. 107].
- 21.
On the rationality group and Picard–Vessiot theory, see Gray’s historical account [255, pp. 267ff.].
- 22.
Loewy refers to Schlesinger’s 1908 book on differential equations [514, pp. 157–158], which may have suggested to him the value of this connection for his own work, as described below.
- 23.
The main results of Loewy’s theory are outlined in [427]. His most complete exposition is contained in a paper published in Mathematische Zeitschrift in 1920 and dedicated to Ludwig Stickelberger on the occasion of his 70th birthday [429]. The 1917 paper, however, brings out more fully the connections with Frobenius’ theory.
- 24.
- 25.
Krull’s companion matrices were defined as the negatives of Loewy’s because Krull defined the characteristic polynomial as det(t I − A), rather than as det(t I + A), as with Loewy.
- 26.
These papers are [4]–[7] in Krull, Abhandlungen 1. For an overview of their contents, as well as Krull’s subsequent work on the theory of ideals, see P. Ribenboim’s essay on Krull [494, pp. 3ff.].
- 27.
Despite his considerable mathematical talent, Remak, a Jew, did not have a correspondingly successful career as a mathematician and was eventually deported to Auschwitz, where he perished. For more on Remak’s life and work, see [440].
- 28.
In the terminology and notation of Remak, \(\mathfrak{H}\) is the direct product \(\mathfrak{H} = \mathfrak{A} \times \mathfrak{B}\) of nontrivial subgroups \(\mathfrak{A}\), \(\mathfrak{B}\) if (1) every \(g \in \mathfrak{G}\) is expressible as g = a b, with \(a \in \mathfrak{A}\) and \(b \in \mathfrak{B}\); (2) \(\mathfrak{A} \cap \mathfrak{B} =\{ E\}\); (3) every \(a \in \mathfrak{A}\) commutes with every \(b \in \mathfrak{B}\). When \(\mathfrak{H}\) is abelian, this agrees with the definition of \(\mathfrak{H} = \mathfrak{A} \times \mathfrak{B}\) given by Frobenius and Stickelberger in 1878. Condition (3) must be added for nonabelian groups.
- 29.
An automorphism σ of \(\mathfrak{H}\) is central if h − 1 σ(h) is in the center of \(\mathfrak{H}\) for all \(h \in \mathfrak{H}\) [493, p. 293], a notion still in use today.
- 30.
The ascending chain condition for ideals had been introduced by Noether in her above-mentioned fundamental paper of 1921 [455, Satz I, p. 30] and goes back to Dedekind, as she noted. All Krull’s early papers on ideals cite this paper as a basic reference. Nine months before submitting his paper [375] on generalized abelian groups, Krull had submitted a paper with a descending chain condition on the successive powers of an ideal [374, p. 179, (f)]. As for Noether, her important work on what are now called Dedekind rings [456, 457], involved a descending chain condition. A bit later, Artin used a descending chain condition in his study of what are now called Artinian rings.
- 31.
Krull spoke simply of indecomposable subgroups, but he meant the analogue of what Remak had called “directly indecomposable” subgroups.
- 32.
See [375, p. 191 n.34]. Krull was referring to Loewy’s results on matrix complexes (described above) as they apply to the completely reducible operators of Loewy’s Theorem 16.8. Of course, it should be kept in mind that although Krull’s theorem applies to any \(\mathfrak{A}\), the \(\mathfrak{D}_{ii}\) are simply directly indecomposable and not necessarily irreducible in Loewy’s sense.
- 33.
Krull did not use the terminology of linear transformations and vector spaces, which was soon brought into linear algebra primarily through the influence of the work of Weyl (see below).
- 34.
- 35.
Lasker reserved the term “ideal” for an ideal in Z[x 1, …, x n ].
- 36.
Châtelet also extended his notion of module to “smireal” n-dimensional spaces [88, pp. 10, 25].
- 37.
In §§106–107, van der Waerden also allowed \(\mathfrak{R}\) to be what one might call a “noncommutative Euclidean domain” [568, p. 120].
- 38.
That is, detU, which belongs to \(\mathfrak{R}\), has an inverse in \(\mathfrak{R}\). Thus \({U}^{-1} = {(\det U)}^{-1}\mathrm{Adj}\,U\) is a matrix over \(\mathfrak{R}\), and \(x = u{U}^{-1}\).
- 39.
Sperner explained in the preface that the subject matter of the book was as Schreier intended, although the ordering of the material and some proofs were changed in part to achieve greater simplicity.
- 40.
In Space–Time–Matter, first published in 1921, Weyl developed the machinery of tensor algebra within the context of abstract vector spaces. In his 1923 monograph Mathematical Analysis of the Space Problem [601, Anhang 12], Weyl developed elementary divisor theory over \(\mathbb{C}\) within the same context and so made the concept of a linear transformation acting on a vector space fundamental. For more on Weyl and the space problem, see [518, §2.8] and [276, §11.2].
- 41.
For a discussion of Schreier’s important work on continuous groups, see [276, pp. 497ff.].
- 42.
- 43.
As with Theorem 16.11, \(\mathfrak{R}\) can also be a “noncommutative Euclidean domain.” See the citation at that theorem.
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Hawkins, T. (2013). Loose Ends. In: The Mathematics of Frobenius in Context. Sources and Studies in the History of Mathematics and Physical Sciences. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-6333-7_16
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