# Abstract Algebra 1

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 Author: collin_e ID: 131837 Filename: Abstract Algebra 1 Updated: 2012-01-30 21:03:46 Tags: Math Abstract Algebra Folders: Description: First test review Show Answers:

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1. Definition of the Cartesian product of two sets.
Let A and B be sets. The set A x B = {(a,b) | a A and b B} is the cartesian product of A and B.
2. Definition a function and related concepts.
A function f mapping X onto Y is a relation between X and Y with the property that each x X appears as the first member of exactly one ordered pair (x,y) in f. Such a function is also called a map of mapping of X into Y. We write f:X->Y and express (x,y) f by f(x)=y. The domain of f is the set X and the set Y is the codomain of f. The range of f is f[X] = {f(x) | x X}.
3. Definition of injective (one-to-one), surjective (onto), and bijective function.
A function f:X->Y is one to one if f(x1)=f(x2) only when x1=x2. The function f is onto Y if the range of f is Y. The function f is bijective if it is one to one and onto.
4. Definition of two sets with the same cardinality.
Two sets X and Y have the same cardinality if there exists a one-to-one function mapping X onto Y, that is, if there exists a one-to-one correspondence between X and Y.
5. Prove that the sets Z and Q have cardinality .
For each n N, let An be the set given by An = {p/q | where p,q N are in lowest terms with p + q = n}. Each An is finite and every rational number appears in exactly one of these sets. A countable union of finite sets is countable.
6. Definition of partition.
A partition of a set S is a collection of nonempty subsets of S such that every element of S is in exactly one of the subsets. The subsets are the cells of the partition.
7. Definition of an equivalence relation.
• An equivalence relation R on a set S is one that satisfies these three properties for all x, y, z S.
• 1. (Reflexive) xRx.
• 2. (Symmetric) If xRy, then yRx.
• 3. (Transitive) If xRy and yRz, then xRz.
8. Define Congruuence Modulo n.
Let n Z+. The equivalence relation on Z+ corresponding to the partition of Z+ into residue classes modulo n is congruence modulo n. It is denoted a =n b or a = b (mod 4), read "a is congruent to b modulo n."
9. (Equivalence Relations and Partitions) Let S be a nonempty set and let ~ be an equivalence relation on S. Then ~ yields a partition of S, where = {x S | x ~ a}. Also, each partition of S gives rise to an equivalence relation ~ on S where a ~ b if and only if a and b are in the same cell of the partition.
We must show that the different cells = {x S | x ~ a} for a S do give a partition of S, so that every element of S is in some cell and so that if a , then = . Let a S. Then a by the reflexive condition, so a is in at least one cell. Suppose now that a were in a cell also. We need to show that = as sets; this will show that a cannot be in more than one cell. There is a standard way to show that two sets are the same: Show that each set is a subset of the other. We show that . Let x . Then x ~ a. But a , so a ~ b. Then, by the transitive condition, x ~ b, so x . Thus . Now we show that . Let y B. Then y ~ b. But a , so a ~ b and by symmetry b ~ a. Then by transitivity y ~ a, so y . Hence also, so = and our proof is complete.

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