# Ideal Theory by D. G. Northcott

By D. G. Northcott

Perfect concept is necessary not just for the intrinsic curiosity and purity of its logical constitution yet since it is an important instrument in lots of branches of arithmetic. during this advent to the trendy idea of beliefs, Professor Northcott assumes a legitimate historical past of mathematical conception yet no prior wisdom of contemporary algebra. After a dialogue of trouble-free ring thought, he bargains with the homes of Noetherian jewelry and the algebraic and analytical theories of neighborhood jewelry. in an effort to supply a few suggestion of deeper purposes of this thought the writer has woven into the attached algebraic concept these effects which play amazing roles within the geometric purposes.

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2. Let D = Z, the integers, p = 13. Then D mod p has 13 elements, which we may denote by 0, I, ... , 12. Then for example Let us find the inverse of 6. We apply Euclid's algorithm to 6 and 13 to find a linear combination of 6 and 13 equal to 1. We find that 6 . 11 - 13 . 5 = 1. , (6)- = 11. 3. Take D = Z again, but now let p be an arbitrary prime. The resulting important finite field has exactly p elements {O, I, ... , p - l}j it is commonly denoted by either of the two symbols Fp or GF(p}. This construction yields infinitely many finite fields, since there are infinitely many • primes.

6 we know that this polynomial will factor uniquely into a product of irreducible monic polynomials over k. The next theorem tells us something more about this factorization. 1. xqn - X = II Vd(X), din where Vd(X) is the product of all monic irreducible polynomials in k[x] of degree d. Proof: Let d be a divisor of n and let f(x) be a monic irreducible polynomial of degree dover k. Form the field F = k[x] (modf(x))j then F has qd elements. , a = x. 10, a qd = a, and this is equivalent to the statement d f(x) I (x q - x).

Let a k = ak+t be the first repeat in the sequence. Then clearly k = OJ otherwise a k- l = ak+t-l would be an earlier repeat. Thus (1, a, ... , at-I) are all distinct, but at = 1. The integer t ~ 1 is called the order of a. This number will in general be different for different values of a; and given an element a, it may be difficult to calculate t. However, it turns out that we can say exactly how many elements of each order t ~ 1 are contained in F. Our first step in this direction is a special case of a famous theorem of Lagrange.

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