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# Class Note for MATH 300 at UMass(7)

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Date Created: 02/06/15
Math 300 Fall 2008 Note 1 Zhigang Han7 Umass at Amherst 1 Logic and Proofs 11 Logic Def 1 A statement or proposition is a sentence that is either TRUE or False Def 2 Let P and Q be statements 1 The logical conjunction P A Q P AND Q is a statement that is TRUE if both P and Q are TRUE7 and is FALSE otherwise 2 The logical disjunction P V Q P QR Q is a statement that is TRUE if either P is TRUE or Q is TRUE or both are TRUE7 and is FALSE otherwise 3 The logical negation N P NOT P is a statement that is TRUE if P is FALSE7 and is FALSE if P is TRUE 4 The logical implication or conditional statement P a Q If P then Q is a statement that is FALSE if P is TRUE and Q is FALSE7 and is TRUE otherwise Rmk In the de nition of the disjunction P V Q7 OR is inclusive ln fact7 we always use inclusive OR in mathematics Try to give yourself examples of both inclusive OR and exclusive OR in English Def 3 A truth table is a table which lists the truth values of a compound statement resulting from all possible combinations of the truth values of its components Rmk If a compound statement has 71 unknown components7 then there are 2 possible combinations Thus the truth table has 2 rows Eg 1 Truth tables for conjunction disjunction negation and implication Def 4 Two statements are logically equivalent if they have the same truth table Eg 2 Show that N P V Q and N P A N Q are equivalent Eg 3 Show that P gt Q and N P V Q are equivalent Def 5 The converse of P gt Q is Q gt P The inverse of P gt Q is N P gt N Q The contrapositive of P gt Q is N Q gt N P Eg 4 Show that P gt Q is equivalent to its contrapositive N Q gt N P Rink P gt Q is NOT always equivalent to its converse or inverse Def 6 The statement P Q Q P if and only if Q or simply P iff Q is de ned by P gt Q A Q gt P Rink If P gt Q we often say P is suf cient for Q and Q is necessary for P If P Q Q we say P is necessary and suf cient for Q Eg 5 Use common sense to nd the truth table for P Q Q then use the de nition of P Q Q to verify it Rink Note that P Q Q is TRUE if and only if P and Q have the same truth values ie P and Q are logically equivalent Eg 6 We can restate Eg 3 as P gt Q Q P V Q Prove this by constructing its truth table 12 Sets A set is a collection of objects the objects are called the elements or members of the set Def 1 If a is an element of the set S we say a belongs to S and write a E S If I is not an element of S we write 1 S One can describe a set by listing all its elements or by means of rules For instance A 124 and B ml1lt z lt 5 Standard notations 1 N 1 2 34 is the set of all natural numbers 2 Z 72 10 1 2 is the set of all integers 3 Q is the set of all rational numbers 4 R is the set of all real numbers Def 2 Let A and B be two sets 1 We say A is a subset of B denoted by A Q B or B Q A if z E A implies z E B 2 We say A and B are equal sets denoted by A B if A Q B and B Q A 3 We say A is a proper subset of B denoted by A C B or B D A if A is a subset of B but A is not equal to B 4 The intersection of A and B is de ned by A B xlm AAx B If A O B 0 we say A and B are disjoint 5 The union of A and B is de ned by AUB xlm AVx B 6 The Cartesian product of A and B is de ned by AXB zylm AAy B Eg 11fA 12 and B 23 then A B 2 Au B 123 and A X B 17 2 173 272 273 Eg 2 A set is called an empty set if it contains no elements Show that all empty sets are equal We denoted the unique empty set by 0 Eg 3 Prove that the empty set is a subset of any other set Eg 4 Let A and B be any two sets Show that A O B Q A U B 13 Quanti ers Def 1 Let Pm be a statement depending on m 1 The universal quanti cation of Pm is a statement 0 Pm for all values of m and is denoted by 0 Vm Here the symbol Vz for all is called the universal quanti er 2 The existential quanti cation of Pm is a statement 0 Pm for some value of m and is denoted by 0 3x Here the symbol Hz there exists is called the existential quanti er Rink The values of z is assumed to be in a particular set7 called the universe of discourse For instance7 it can be N Z or R Or we can directly specify the set S by stating as follows 0 Vm E S 0 3m 6 S Eg 1 The universal quanti cation Vz E R z2 gt 0 is FALSE7 but Vz E R x2 Z 0 is TRUE Eg 2 The existential quanti cation Hz 6 R z2 lt 0 is FALSE7 but Hz 6 R x2 g 0 is TRUE Quanti er negation rules 0 N Vz7 and 3m N Pm are equivalent 0 N Hz and Vm N Pm are equivalent Eg 3 Assume the universe of discourse is Z What do the following state ments mean in English Are they TRUE Can you negate them 0 Vm 3y x gt 0 3y Vz m 2 14 Counterexam ples Def 1 A conjecture is a result which is thought to be TRUE but has not been proven Rink A conjecture is often in the form of a universal quanti cation Vm Note that a conjecture is not necessarily TRUE One can either try to prove it or disprove it 1 To prove Vm Pz7 one has to show Pm is TRUE for all m 2 To disprove Vm Pz7 we need to show N Vz7 is TRUE By the quanti er negation rules7 N Vz7 is equivalent to 3m N There fore7 we only have to nd one value c such that Pm is FALSE This value c is called a counterexample Eg 1 Prove or disprove All prime numbers are odd Question How to disprove a conjecture Vm Pm gt Qm7 Answer Find a value c such that Pc is TRUE7 and 620 is FALSE Why N V907 PM QM ltgt Hz N a ltgt 3957 PM A N QM The 2nd step uses that fact that N P a Q is equivalent to P A N Q Verify this using truth tables Eg 2 Prove or disprove All continuous functions from R to R are differ entiable 15 Proofs A compound statement P can be TRUE or FALSE To prove P means to prove P is TRUE Now let s try to understand some familiar proof methods 0 Direct proof of P a Q Method Assume P is TRUE7 and use this to prove Q is TRUE Question Why can we assume P is TRUE Eg 1 Prove that ifA B A7 then A Q B 0 Direct proof of P ltgt Q Method Prove both P a Q and Q gt P Rink This is the de nition of P ltgt Q Eg 2 Prove that A O B A if and only ifA Q B o Contrapositive proof of P a Q Method Assume Q is FALSE7 and use it to prove P is FALSE Rink P a Q and N Q a N P are equivalent Eg 3 Show that if z E R satis es 2008 z 1 g 37 then x g 1 o Prove P by contradiction Method Assume P is FALSE7 and use this to deduce a contradiction Rink A contradiction has a FALSE value7 and N P a F is equivalent to P Eg 4 Prove that xi is irrational Eg 5 There are 15 people in a party Some of them exchange their handshakes with some of the others Show that at least two people have shaken the same number of hands o Prove P gt Q by contradiction Method Assume P is TRUE and Q is FALSE7 and use them to deduce a contradiction Rink P a Q and P A N Q a F are equivalent Eg 6 Prove the following Pigeonhole Principle by contradiction Pigeonhole principle If N 2 km 1 pigeons are placed in k pigeonholes7 then at least one pigeonhole contains at least m 1 pigeons Eg 7 In a class of 30 students7 show that there are at least 3 students Whose birthday are in the same month Eg 8 Place randomly 5 points in a square of area 16 ftz Show that there are at least 2 points Whose distance from each other is less than 3 ft o Prove P a Q VR Method Assume P is TRUE and Q is FALSE7 and use them to prove R is TRUE Rink P a Q V R and P A N Q a R are equivalent o Prove P VQ R Method Assume P is TRUE and use it to prove R is TRUE Then Assume Q is TRUE and use it to prove R is TRUE Rink P V Q a R and P a R A Q a R are equivalent Eg 9 Let 71 6 R Show that my 0 if and only ifz 0 or y O o Prove P a Q AR Method Assume P is TRUE7 and use it to prove Q and R are both TRUE Rink P a Q A R and P a Q A P a R are equivalent o Prove P AQ R Method Assume P is TRUE and Q is TRUE7 and use them to prove R is TRUE Eg 10 Let m7 n E Z Show that mm is odd if and only if m is odd and n is odd

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