Let R be an integral domain and I a nonzero ideal of R. An ideal J ⊆ I is a t-reduction of I if (JI n ) t = (I n+1 ) t for some integer n ≥ 0. An element x ∈ R is t-integral over I if there is an equation x n + a 1 x n−1 + ... + a n−1 x + a n = 0 with a i ∈ (I i ) t for i = 1, ..., n. The set of all elements that are t-integral over I is called the tintegral closure of I. This paper investigates the t-reductions and t-integral closure of ideals. Our objective is to establish satisfactory t-analogues of well-known results, in the literature, on the integral closure of ideals and its correlation with reductions. Namely, Section 2 identifies basic properties of t-reductions of ideals and features explicit examples discriminating between the notions of reduction and t-reduction. Section 3 investigates the concept of t-integral closure of ideals, including its correlation with t-reductions. Section 4 studies the persistence and contraction of t-integral closure of ideals under ring homomorphisms. All along the paper, the main results are illustrated with original examples. 1
t-Reductions of idealsThis section identifies basic ideal-theoretic properties of the notion of t-reduction including its behavior under localizations. As a prelude to this, we provide explicit examples discriminating between the notions of reduction and t-reduction.Recall that, in a ring R, a subideal J of an ideal I is called a reduction of I if JI n = I n+1 for some positive integer n [23]. An ideal which has no reduction other than itself is called a basic ideal [12,13].Definition 2.1 (cf. [15, Definition 1.1]). Let R be a domain and I a nonzero ideal of R. An ideal J ⊆ I is a t-reduction of I if (JI n ) t = (I n+1 ) t for some integer n ≥ 0 (and, a fortiori, the relation holds for n ≫ 0). The ideal J is a trivial t-reduction of I if J t = I t . The ideal I is t-basic if it has no t-reduction other than the trivial t-reductions.At this point, recall a basic property of the t-operation (which, in fact, holds for any star operation) that will be used throughout the paper. For any two nonzero ideals I and J of a domain, we have (IJ) t = (I t J) t = (IJ t ) t = (I t J t ) t . So, obviously, for nonzero ideals J ⊆ I, we always have:Notice also that a reduction is necessarily a t-reduction; and the converse is not true, in general, as shown by the next example which exhibits a domain R with two t-ideals J I such that J is a t-reduction but not a reduction of I. Example 2.2. We use a construction from [18]. Let x be an indeterminate over Z and let R := Z[3x, x 2 , x 3 ], I := (3x, x 2 , x 3 ), and J := (3x, 3x 2 , x 3 , x 4 ). Then J I are two finitely generated t-ideals of R such that: JI n I n+1 ∀ n ∈ N and (JI) t = (I 2 ) t .