Lecture 4.5: POSets and Hasse Diagrams CS 250, Discrete Structures, Fall 2011 Nitesh Saxena *Adopted from previous lectures by Cinda Heeren

Lecture POSets and Hasse Diagrams Course Admin HW4 has been posted Covers the chapter on Relations (lecture 4.*) Due at 11am on Nov 16 (Wednesday) Also has a 10-pointer bonus problem Please start early

Final Exam Thursday, December 8, 10:45am- 1:15pm, lecture room Heads up! Please mark the date/time/place Our last lecture will be on December 6 We plan to do a final exam review then Lecture POSets and Hasse Diagrams

Outline Hasse Diagrams Some Definitions and Examples Maximal and miminal elements Greatest and least elements Upper bound and lower bound Least upper bound and greatest lower bound

Hasse Diagrams Hasse diagrams are a special kind of graphs used to describe posets. Ex. In poset ({1,2,3,4},  ), we can draw the following picture to describe the relation. 1.Draw edge (a,b) if a  b 2.Don’t draw up arrows 3.Don’t draw self loops 4.Don’t draw transitive edges Lecture POSets and Hasse Diagrams

Hasse Diagrams Have you seen this one before? String comparison poset from last lecture

Lecture POSets and Hasse Diagrams Maximal and Minimal Consider this poset: Reds are maximal. Blues are minimal.

Maximal and Minimal: Example Q: For the poset ({2, 4, 5, 10, 12, 20, 25}, |), what is/are the minimal and maximal? A: minimal: 2 and 5 maximal: 12, 20, 25 Lecture POSets and Hasse Diagrams

Least Element and Greatest Element Definition: In a poset S, an element z is a minimum (or least) element if  b  S, z  b. Write the defn of maximum (geatest)! Did you get it right? Intuition: If a is maxiMAL, then no one beats a. If a is maxiMUM, a beats everything. Must minimum and maximum exist? A.Only if set is finite. B.No. C.Only if set is transitive. D.Yes.

Maximal and Minimal: Example Q: For the poset ({2, 4, 5, 10, 12, 20, 25}, |), does the minimum and maximum exist? A: minimum: [divisor of everything] No maximum: [multiple of everything] No Lecture POSets and Hasse Diagrams

A Property of minimum and maximum Theorem: In every poset, if the maximum element exists, it is unique. Similarly for minimum. Proof: Suppose there are two maximum elements, a 1 and a 2, with a 1  a 2. Then a 1  a 2, and a 2  a 1, by defn of maximum. So a 1 =a 2, a contradiction. Thus, our supposition was incorrect, and the maximum element, if it exists, is unique. Similar proof for minimum.

Lecture POSets and Hasse Diagrams Upper and Lower Bounds Defn: Let (S,  ) be a partial order. If A  S, then an upper bound for A is any element x  S (perhaps in A also) such that  a  A, a  x. Ex. The upper bound of {g,j} is a. Why not b? A lower bound for A is any x  S such that  a  A, x  a. ab d j f i h e c g Ex. The upper bounds of {g,i} is/are… A. I have no clue. B. c and e C. a D. a, c, and e {a, b} has no UB.

Lecture POSets and Hasse Diagrams Upper and Lower Bounds Defn: Let (S,  ) be a partial order. If A  S, then an upper bound for A is any element x  S (perhaps in A also) such that  a  A, a  x. Ex. The lower bounds of {a,b} are d, f, i, and j. A lower bound for A is any x  S such that  a  A, x  a. ab d j f i h e c g Ex. The lower bounds of {c,d} is/are… A. I have no clue. B. f, i C. j, i, g, h D. e, f, j {g, h, i, j} has no LB.

Lecture POSets and Hasse Diagrams Least Upper Bound and Greatest Lower Bound Defn: Given poset (S,  ) and A  S, x  S is a least upper bound (LUB) for A if x is an upper bound and for upper bound y of A, x  y. Ex. LUB of {i,j} = d. x is a greatest lower bound (GLB) for A if x is a lower bound and if y  x for every lower bound y of A. ab d j f i h e c g Ex. GLB of {g,j} is… A. I have no clue. B. a C. non-existent D. e, f, j

Lecture POSets and Hasse Diagrams LUB and GLB Ex. In the following poset, c and d are lower bounds for {a,b}, but there is no GLB. Similarly, a and b are upper bounds for {c,d}, but there is no LUB. ab d c This is because c and d are incomparable.

Another Example What are the GLB and LUB, if they exist, of the subset {3, 9, 12} for the poset (Z +, |)? What are the GLB and LUB, if they exist, of the subset {1, 2, 4, 5, 10} for the poset (Z +, |) Lecture POSets and Hasse Diagrams

Another Example What are the GLB and LUB, if they exist, of the subset {3, 9, 12} for the poset (Z +, |)? LUB: [least common multiple] 36 GLB: [greatest common divisor] 3 What are the GLB and LUB, if they exist, of the subset {1, 2, 4, 5, 10} for the poset (Z +, |) LUB: [least common multiple] 20 GLB: [greatest common divisor] 1 Lecture POSets and Hasse Diagrams

Example to sum things up For the poset ({2, 4, 6, 9, 12, 18, 27, 36, 48, 60, 72}, |), find the following: 1. Maximal element(s) 2. Minimal element(s) 3. Greatest element, if it exists 4. Least element, if it exists 5. Upper bound(s) of {2, 9} 6. Least upper bound of {2, 9}, if it exists 7. Lowe bound(s) of {60, 72} 8. Greatest lower bound of {60, 72}, if it exists Lecture POSets and Hasse Diagrams

Example to sum things up For the poset ({2, 4, 6, 9, 12, 18, 27, 36, 48, 60, 72}, |), find the following: 1. Maximal element(s) [not divisors of anything] 27, 48, 60, Minimal element(s) [not multiples of anything] 2, 9 3. Greatest element, if it exists [multiple of everything]No 4. Least element, if it exists [divisor of everything]No 5. Upper bound(s) of {2, 9} [common multiples]18, 36, Least upper bound of {2, 9}, if it exists [least common multiple]18 7. Lower bound(s) of {60, 72} [common divisors]2, 4, 6, Greatest lower bound of {60, 72}, if it exists [greatest common divisor] 12 Lecture POSets and Hasse Diagrams

More Theorems Theorem: For every poset, if the LUB for a set exist, it must be unique. Similarly for GLB. Proof: Suppose there are two LUB elements, a 1 and a 2, with a 1  a 2. Then a 1  a 2, and a 2  a 1, by defn of LUB. So a 1 =a 2, a contradiction. Thus, our supposition was incorrect, and the LUB, if it exists, is unique. Similar proof for GLB.

Lecture POSets and Hasse Diagrams Today’s Reading Rosen 9.6