Algebraic theories by Dickson, Leonard Eugene

By Dickson, Leonard Eugene

This in-depth advent to classical subject matters in greater algebra presents rigorous, specified proofs for its explorations of a few of arithmetic' most vital recommendations, together with matrices, invariants, and teams. Algebraic Theories stories all the very important theories; its vast choices variety from the rules of upper algebra and the Galois idea of algebraic equations to finite linear groups  Read more...

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Eliminating X 4 + Y4 between the expressions for / and H, we get A2 m f - H = A2(9m2 - 1 )X 2 X2. This gives X y by extracting the square root of a form which is a perfect square. For, if 9m2 = 1, / would be the square of X 2 ± Y2, contrary to the hypothesis that there is no multiple root. From X Y a n d /, we get X 2 + Y2 and then X ± Y and hence X and Y. T heorem 18. Every binary quartic form f (x, y) not divisible by the square of a linear form can be reduced to a canonical form (23). We canfind m and the linearfunctions X and Y of x and y by finding a root r of a cubic equation (24), whose coefficients are the invariants I and J of / , and by extracting the square root of rf — H, where 144# is the Hessian of / .

T heorem 7. Every sufficiently general binary form of odd order 2n — 1 is a sum of (2n — 1 )th powers of n linear forms. E xercises 1. For n = 3, the coefficient of x3 in the canonizant C of the quintic / is the invariant J of the quartic q = a0x4 + 4 a i xzy + . . In particular, J is a seminvariant of q and hence is annihilated by the operator 12 in (6) with p = 4. Since J lacks o5 it is therefore annihilated by 12 with p = 5 and hence is a seminvariant of f. 2. Show that J is a seminvariant of every / with p ^ 4 since it is a rational function of the seminvariants a0, S2 , S3,

Zm, we may take zx = Ih . . , zm = I m. T heorem 6. There is a finite number of syzygies Si = 0, . . , Sk = 0, such that every syzygy S = 0 implies a relation (15) in which P i , . . , Pk are now invariants. In particular, every syzygy is a consequence of Si = 0, . . , Sk = 0. By the Lemma in §18, the proof holds also for syzygies between covariants. 36 COYARIANTS OF BINARY FORMS [Ch. II 20. Canonical form of a binary form of odd order. In §7 we saw that any binary cubic whose discriminant is not zero is the sum of the cubes of two Linear forms which are the factors of the Hes­ sian.

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