# Algebra by Dan Laksov

By Dan Laksov

Similar elementary books

Understanding elementary algebra with geometry: a course for college students, 6th Edition

Hirsch and Goodman provide a mathematically sound, rigorous textual content to these teachers who think scholars can be challenged. The textual content prepares scholars for destiny research in higher-level classes by means of progressively construction scholars' self assurance with out sacrificing rigor. to assist scholars circulation past the "how" of algebra (computational talent) to the "why" (conceptual understanding), the authors introduce subject matters at an ordinary point and go back to them at expanding degrees of complexity.

Example text

If s and t are sections of F (U ) such that sx = tx for all x ∈ U , then, for each x ∈ U , U there exists a neighbourhood Ux of x contained in U such that ρU Ux (s) = ρUx (t). Since the sets Ux for all x ∈ U cover U it follows from property (F1) for sheaves that s = t. Hence property (i) holds. The images by πU of the sections of F (U ) satisfy property (ii) of assertion (2) since for s ∈ F (U ) we can take Ux = U and s(x) = s for all x ∈ U . Conversely let (sx )x∈U ∈ x∈U Fx satisfy the condition of (ii), We shall show that (sx )x∈U is in the image of πU .

Let K be a field and let A = K n be the product of the field K with itself n times. (1) Describe Spec(A). (2) What is the dimension of Spec(A)? 7. Let A = Zn be the cartesian product of the ring Z with itself n times. (1) Describe Spec(A). (2) What is the dimension of Spec(A)? 8. A topological space X is connected if there is no non-empty subset of X different from X. Let A be a ring and let X = Spec(A). Show that the following assertions are equivalent: (1) The space X = Spec(A) is not connected.

F2) For every collection {sα }α∈I of sections sα ∈ F (Uα ) that satisfy the condition U β α ρU V (sα ) = ρV (sβ ) for all α, β in I, and all V belonging to B with V ⊆ Uα ∩Uβ , there is a section s in F (U ) restricting to Uα for all α ∈ I, that is, ρU Uα (s) = sα for all α ∈ I. A presheaf, or sheaf, that is defined on all the open subsets of X is called a presheaf respectively a sheaf on X. 2) Remark. If follows from property (F1) that the section s of property (F2) i unique. Moreover from the equality ∅ = ∅ ∪ ∅ is follows from property (F2) for sheaves that when not all the F (U ) with U belonging to B are empty we have that F (∅) consists of exactly one element.