230 likes | 466 Views
One-to-One and Onto, Inverse Functions. Lecture 31 Section 7.2 Thu, Apr 6, 2006. Four Important Properties. Let R be a relation from A to B . R may have any of the following four properties. x A , at least one y B , ( x , y ) R .
E N D
One-to-One and Onto, Inverse Functions Lecture 31 Section 7.2 Thu, Apr 6, 2006
Four Important Properties • Let R be a relation from A to B. • R may have any of the following four properties. • x A, at least one y B, (x, y) R. • x A, at most one y B, (x, y) R. • y B, at least one x A, (x, y) R. • y B, at most one x A, (x, y) R.
One-to-one and Onto • R is onto if • y B, at least one x A, (x, y) R. • R is one-to-one if • y B, at most one x A, (x, y) R.
Combinations of Properties • R is a function if • x A, at exactly one y B, (x, y) R. • R is one-to-one and onto if • y B, at exactly one x A, (x, y) R.
Examples • Consider the following “functions.” • f : RR by f(x) = 2x. • g : RR by g(x) = 1/x. • h : RR by h(x) = x2. • k : RR by m(x) = x. • m : QQ by k(a/b) = a.
Examples • Which of the five examples are functions? • Prove that g is one-to-one, but not onto. • Is it one-to-one and onto from Q – {0} to Q – {0}? • Prove that h is neither one-to-one nor onto. • What about k? • What about m?
One-to-one Correspondences • A function f : AB is a one-to-onecorrespondence if f is one-to-one and onto. • f has all four of the basic properties. • f establishes a “pairing” of the elements of A with the elements of B.
One-to-one Correspondences • One-to-one correspondences are very important because two sets are considered to have the same number of elements if there exists a one-to-one correspondence between them.
Example: One-to-one Correspondence • Are any of the following “functions” one-to-one correspondences? • f : RR by f(x) = 2x. • g : RR by g(x) = 1/x. • h : RR by h(x) = x2. • m : RR by k(x) = x. • Is g a one-to-one correspondence from R – {0} to R – {0}?
Inverse Relations • Let R be a relation from A to B. • The inverse of R is the relation R–1 from B to A defined by the property that (x, y) R–1 if and only if (y, x) R.
Example: Inverse Relation • Let f : RR by f(x) = 2x. • Describe f –1. • f –1(x) = x/2. • Which of the four basic properties does f –1 have?
Example: Inverse Relation • Let g : RR by g(x) = 1/x. • Describe g–1. • g–1(x) = 1/x. • Which of the four basic properties does g–1 have?
Example: Inverse Relation • Let h : RR by h(x) = x2. • Describe h–1. • Which of the four basic properties does h–1 have?
Example: Inverse Relation • Let k : RR by k(x) = x. • Describe k–1. • Which of the four basic properties does k–1 have?
Inverse Relations and the Basic Properties • A relation R has the first basic property if and only if R–1 has the third basic property. • x A, at least one y B, (x, y) R. • y B, at least one x A, (x, y) R.
Inverse Relations and the Basic Properties • A relation R has the second basic property if and only if R–1 has the fourth basic property. • x A, at most one y B, (x, y) R. • y B, at most one x A, (x, y) R.
Inverse Functions • Theorem: The inverse of a function is itself a function if and only if the function is a one-to-one correspondence. • Corollary: If f is a one-to-one correspondence, then f –1 is a one-to-one correspondence. • The inverse of a function is, in general, a relation, but not a function.
Example: Inverse Functions • Let A = R – {1/3}. • Let B = R – {2/3}. • Define f : AB by f(x) = 2x/(3x – 1). • Find f –1. • Let y = 2x/(3x – 1). • Swap x and y: x = 2y/(3y – 1). • Solve for y: y = x/(3x – 2). • Therefore, f –1(x) = x/(3x – 2).
Q and Z • Theorem: There is a one-to-one correspondence from Z to Q. • Proof: • Consider only rationals in reduced form. • Arrange the positive rationals in order • First by the sum of numerator and denominator. • Then, within groups, by numerator.
Q and Z • The first group: 1/1 • The second group: 1/2, 2/1 • The third group: 1/3, 3/1 • The fourth group: 1/4, 2/3, 3/2, 4/1 • Etc. • The sequence is 1/1, 1/2, 2/1, 1/3, 3/1, 1/4, 2/3, 3/2, 4/1, …
Q and Z • Let f : ZQ be the function that • Maps the positive integer n to the nth rational in this list. • Maps the negative integer -n to the negative of the rational that n maps to. • Maps 0 to 0. • This is a one-to-one correspondence.
Q and Z • What is f(20)? • What is f –1(4/5)?