By Daniel B. Shapiro

The important subject of this ebook is the concept of Hurwitz and Radon referring to composition formulation for sums of squares, first proved within the 1920's. options from algebra and topology are used to generalize that theorem in different instructions. The textual content comprises labored examples and lots of workouts, which enhance nonetheless extra diversifications of the principal subject. the most viewers is those that have had a few graduate classes in summary algebra, yet many sections of the publication are available to somebody with a few education in linear algebra. numerous significant themes during this e-book can be of curiosity to scholars of topology and geometry. the writer has tried to make the presen-tation as transparent and as trouble-free as attainable.

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**Extra resources for Compositions of Quadratic Forms (De Gruyter Expositions in Mathematics)**

**Example text**

11. 6. Further properties of δ(s, t) appear in Exercise 3. 2. 14) implies that Sim(V , q) admits a large (s, t)-family. We investigate the converse. 10 subspaces of Sim(q) provide certain Pfister forms which are tensor factors of q. The following consequence of the Eigenspace Lemma is a similar sort of result: certain (s, t)- families in Sim(q) provide Pfister factors of q. 16 Corollary. Suppose (σ ⊥ α, τ ⊥ α) < Sim(q) where dim σ ≥ 1 and α a1 , . . , ak ⊗ γ for some quadratic form γ such that a1 , .

Kb } and define y = h1 h2 . . ha k1 . . kb . Then y˜ = y and y commutes with elements of S and T , and anticommutes with elements of H and K. Therefore (S + yK, T + yH ) ⊆ Sim(V , q). The converse follows since the same operation applied again leads back to the original subspaces. 7 Construction Lemma. Suppose (σ, τ ) < Sim(q) where σ represents 1. If a ∈ F • then (σ ⊥ a , τ ⊥ a ) < Sim(q ⊗ a ). Proof. Recall that a = 1, a is a binary form. Let (S, T ) be an (s, t)-family on (V , q) corresponding to (σ, τ ), and express S = F 1V ⊥ S1 .

Spaces of Similarities 37 Exercise 5. (1) In fact if A, B : V × W → F are bilinear forms such that A(x, y) = 0 implies B(x, y) = 0, then B is a scalar multiple of A. This is proved in Rothaus (1978). A version of this result for p-linear maps appears in Shaw and Yeadon (1989). In a different direction, Alpers and Schröder (1991) study maps f : V → V (not assumed linear) which preserve the orthogonality of the vectors in V • (the set of anisotropic vectors). (2) See Samuel (1968), de Géry (1970), Lester (1977).