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# Class Note for CMPSCI 601 at UMass(24)

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Date Created: 02/06/15

CMPSCI 601 Recall From Last Time Lecture 2 De nitions 0 An alphabet is a nonempty nite set eg E 0 1 etc 0 The set of regular expressions 122 over alphabet E 0 A language is regular iff it is denoted by some regular expression 0 A DFA is atuple D Q E 6 s 0 An NFA is atuple N Q 215 s Prop 12 Every NFA N can be translated into an NFA N which has the same number of states but no etransitions st N N Proposition 13 For every NFA N with n states there is a DFA D with at most 2 states st D N Proof Let N QEA 1017 By Proposition 12 may assume that N has no 6 transitions Let D 2 W2 2 6 C10 F 6Sa U Ara TEES F395 QISHF7 01 096 01 Claim For all 71 E 2 5CIow NUMW By induction on 17111203 5 10 10 1561076 1212 k1 wzua Inductively 5 107 u N010 u 5 107Ua 55 102U7a U AU a T 5q0u U AU a TEAQ0U Aqua Therefore D N Theorem 14 Kleene s Th Let A g 2 be any lan guage Then the following are equivalent 1 A D for some DFA D 2 A N for some NFA N wo 6 transitions 3 A N for some NFA N 4 A e for some regular expression 8 5 A is regular Proof Obvious that l gt 2 gt 3 3 gt 2 by Prop 12 2 gt 1 by Prop 13 subset construction 4 lt gt 5 by def of regular 4 gt 3 We show by induction on the number of symbols in the regular expression 8 that there is an NFA N with 8 N e 80 e o H m Union C oncatenation LNLN1LN2 LNLN1LN2 N1 Kleene Star LltNgt ltLltN1gt N 1 3e4LaNz rwnLEA ij ww If E w I j E Aiw n0 intermediate state gt k ajEAMHLJinj 1 k1 k k k at k sz sz U Lik1Lk1k1 Lk1j k L kI kI Let A g 2 be any language De ne the rightequivalence relation NA 0n 2 chy 42gt VwEEwEi lt gt waA 1 NA y iff 1 and y cannot be distinguished by concate nating some string 11 t0 the right of each of them and testing for membership in A Example A1 w 6 ab I 4712 E 0m0d2 ewAlawAlaa bwabwbbb Claim 19 A1 y iff 42 E 79253 mod 2 Proof Suppose 1 A1 y Let w e cwzrcEAl lt gt ywyEA1 Thus 1129 E 79253 mod 2 Suppose 455 E by mod 2 Vwbcw E byw mod 2 Vwcw 6 A1 lt gt gm 6 A1 Thus 1 A1 y um wez1um a w 6 MY 1 4212 b w 6 MY 1 471 0 mod 2 1 mod 2 Exercise Show that for any language A NA is an equivalence relation Recall that an equivalence relation is a binary relation that is re exive symmetric and tran sitive Proof Re exive V51 6 251 NA 19 Let 29111 E 2 be arbitrary cwEl lt gt Ierl V11 6 25011 E A lt gt 2911 E A because 11 was arbitrary chrc V51 6 251 NA 19 because 1 was arbitrary Symmetric V5134 E 229 NA y gt y NA 19 Let 51931 6 2 be arbitrary Suppose 1 NA y VwcwEA lt gt yw EA Vwyw A lt gt 2971 EA ZINAIC Ay gtyAfIJ Way 6 258 NA 2 gt 9 NA 50 10 Transitive Vzcyz E Ec NAyAy NA z gtc NA z Let 19 y z E 2 be arbitrary Suppose 1 NA y y NA z Vwcw E A lt gt gm 6 A Vwyw E A lt gt zw E A Let w E 2 be arbitrary cwEA lt gt waA waA lt gt szA cwEA lt gt szA V11 6 22911 E A lt gt zw E A because 11 was arbitrary QCNAZ NAyJNAZ gtAZ Vrcyz E 251 NA 3 y NA z gt 1 NA zbecause 19 y z were arbitrary 11 CMPSCI 601 Some Proof Methods Lecture 2 0 To prove V5090 let 1 be arbitrary prove 0 conclude V5090 To prove 0 gt 20 assume 0 prove 20 conclude 0 gt 20 0 From 0 20 may conclude 0 20 0 From 0 2p may conclude 0 20 0 To prove 0 assume 10 prove A A conclude 0 12 Sc M41 9 ltgt 451 E 4 mOd 2 13 MyhillNerode Theorem The language A is regular iff NA has a nite number of equivalence classes Fur thermore this number of equivalence classes is equal to the number of states in the minimumstate DFA that ac cepts A Proof Suppose A D for some DFA D QI7 C127 39 7g 7 E7 67 C117 Let Si 2 w I 5C117 w 2 C12 Claim Each Si contained in single NA equivalence class Let 1 y 6 5239 w E 2 be arbitrary 59117 5m 57579117 M7711 575791179 w V0117 21w 139 Z l Nam E F 5w 6 A lt gt 6q1517w6 F lt gt 6q1yw E F lt gt gm 6 A Vwcw E A lt gt gm 6 A 1 NA y Thus there are at most n equivalence classes 14 Conversely suppose that there are nitely many equiva lence classes of NA E1 Em Let be the equivalence class that 1 is in De ne D 2 El Em E 6 e F where F 56 I 56 E A 5050 a M Must show that 6 is well de ned ie 58 9 gt lival Lyell Suppose 1 NA y Vwcw E A lt gt gm 6 A Vwmw E A lt gt yaw E A Thus 5m NA ya Claim 6 c Proof by induction on exercise E D lt gt 6ecEF lt gt MEF lt gt 2961 15 Example Prove that the following language is regular and its minimal DFA has seven states A7 7116 019 I 71711 D7 016E670 67q d 2 NC dmod 7 2 3g dmod 7 Must show D7 A7 exercise and W 75339 E Oilww l i 74471 Letz 74339 6 01 6 be arbitrary Pick d st 3239 d E 0mod 7 Suppose 3339 d E 0rnod 7 3id E 3jdmod7 3239 E 3339 mod 7 15239 E 15339 mod 7 239 E 339 mod 7 gtlt l6 ThusiodEA7jod A7i74A7j 17 Example ShowE a b I n E N is not regular pf Letz 75 j E N be arbitrary We will show that ai 75E aj Let w bi aim E E aj w Z E Thus NE has in nitely many equivalence classes Thus by the MyhillNerode Theorem E is not regular 18 A language homomorphism is a function h 2 gt I St Vim E 2htvy hrvhy 20 Examples h 0123 gt ab M0 2 an M1 7 M2 aba M3 e M012310 aabababaa g ab gt abc 9a a Cbc gbaa cbcaa Notation for function f A gt B sets 5 Q A T g B f0 a I CLES f1T CLEA I 1 ET Example A1 w 6 ab I 50111 E 0mod2 h1A1 w E O123 I 1w 2w E 0mod 2 MA w E a b c I 656 E 0 mod 2 no otherb or c 19 Closure Theorem for Regular Sets Let A B lt 2 be regular languages and let h 2 gt I and g I gt 2 be homomorphisms Then the following languages are regular 1 AU B 2 AB 321 2 A 4 A B 5mm 69 1A Proof 12 Let e A f B Thus eUf AUB eof AB 3 Let D A DFA D Q 2 6 s F LetB Q26 8Q Thus B Z myAmBZu 20 5 Let A e Thus MA he Example 9a a Cbc A ababa gA acbcacbca 21 6 Let A 30 DFA D Q 2 6 s F Let D QF6 8F Wm 5q7 717 Example MO 2 cm M1 b h2 aba M3 e D a g a 12 0 3 D 0 3 a 12 22

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