(50,1ご 織 お 1謝::リ AN APPLICAT10N OF COHEN'S RESULT ON STAR HEttGⅡ T TO THE THEORY OF CONTROL SttRUCTURES TATSUYA MOTOKI Department of =nformation Sc■ ence 工baraki UniVersity Hitachi 316, -1- 」apan Abstract. We app■ y a resu■ t/concept on star height to two prob■ ems in the theory of contro■ structures. First, we genera■ ize Kosarajuts systems REn structures and show, by using °f C° ntro■ Cohents resu■ t on star he■ ght, that the genera■ ized systems constitute a path― wise hierarchy with respect to their expressive powerse second, by using a concept on star height, we sharpen Peterson et a■ .'s resu■ t that RE∞ is path― wise■ y comp■ ete. -3- 1。 INTRODUCT工 ON A■ though prob■ em regu■ ar events have been extens■ ve■ y studied, the of determining the star height of a regu■ ar event is on■ y partia■ ■y so■ ved. A■ ong resu■ ts on star height our concern is Cohenis resu■ t [2]that for a certain fami■ y of regu■ ar events the star heェ ght equa■ s comp■ ex■ ty of an automaton recogn■ z■ a certain measure of ng the event. The theory of contro■ structures arose from Dijkstra [3] who advocated avo■ ding the use of unnecessar■ ■y comp■ ex contro■ structures ■n programs. Contro■ structures have been w■ de■ y studied before 1975 and many theoretica■ resu■ ts have been presented on their expressive powers. For instance, K9saraju [6] introduced RE[n]― structures (。 r REn ChartS in terms 91 Kosaraju)and showed the semantic hierarchy: RE[1]― where Sem RE[2] Sem RE[う ёach ■ine ] Sem RE[4] Sem means that the right system Of contro■ structures has more express■ ve power than the semantic v■ ewpo■ nt. ■eft from the Other notab■ e resu■ ts are rev■ ewed in Leagard et a■ e[7]. Now we can easi■ y see the ■anguagё ―theoretic between regu■ ar express■ ons and program schemes. simi■ arity ェn thiS paper we app■ y Cohenis resu■ t on star height to a prob■ em in the theory of contro■ structures. Exp■ ic■ t■ y speaking, we genera■ ize Kosaraju:s RE[n]― structures and show the path― wise (or ■anguage― theoretic)hierarchy: -4- l pW RE[2]―重型LRE[2+1/2]pw ....。 where each ■ore ■ine means the right system of contro■ exprOsS■ Ve power than the viewpoint. ■eft from the sructures has ■anguage― theoretic In fact, the hierarchy RE[1]pw RE[2]― 理型二RE[う ]― 理型LRE[4]pw fo■ ■ows immediate■ y frOm Kosara]u:s semantic hierarchy. In additiOn, we sharpen Peterson et a■ .'s resu■ t [8]that the RE[∞ ]― structure is ■anguage― theoretica■ ■y comp■ ete. 2c PRELttMttNARIES 2。 1。 FLOWCHARTS AND PROGRAMS A (deterministic)finite incomp■ ete automaton M is a quintup■ e (S,A,δ an a■ phabet, s。 fina■ to S. ,s。 states and The 'F), where s is a finite set of states, A is iS the initia■ state, F⊂ S is the set of ∈s δ ■anguage in the usua■ way. is the transition partia■ function from S ×A recognized by M, denoted by L(M), is defined Let <M,s〉 denote the finite incomp■ ete automaton obtained from M by changing its initia■ state into s. We say that M is reduced if and On■ y if (1)it has neither any dead state nor any inaccessib■ e state from the initia■ state, and (2)s1 / s2 imp■ ies L(く M,sl〉 )/ L(〈 M,s2〉 )f° r any pair of States sl, s2・ In the seque■ , we use three pairwise■ y dis30int infinite -5- . sets of symbo■ s lF, lP and iF l p ∈lP}; and ■ et = F UP U l百 l p ∈ lP}. The set lF means a set Of prinitive actions (e.ge assignment statements), and the set lP means a set Σ of pr■ m■ tive predicateo by The symbo■ Of identity actiOn, denoted is a■ so in lF. λ, A f■ owchart is a finite incomp■ ete automaton (s,A,δ ,s。 'F) satisfying the fo■ ■owing three conditions: (1)A is a finite subset of Σ. (2)For every s∈ s, either I(s,a)iS defined}= lp,コ }for some p∈ lP, I(s,a)iS definedl = lf}for some f∈ lF, or (2。 2)la∈ A (2。 1)la∈ A I(s,a)iS definedl = φ (3)F = Is∈ S I(s,a)iS undefined for every a∈ A}. (2.う )la∈ A . A f■ owchart (S,A,δ ,6。 ,F)is often dё noted by its state graph (S,E,s。 ), where E = 1(s,五 ,t) l δ(s,a)= tl is the set of edges. The c■ ass of programs and the he■ ght of a progralll Q, denoted by h(Q), are defined as fo■ (1)ヽ ■ows: Any e■ ement of lF, exit(i)for any positive integer i, and halt are programs of height O。 (2)For any p∈ lP UiF l p Q;R and ∈ lPl and prOgrams Q and R, if p then Q e■ se R fi are programS of height max(h(Q),h(R)), and whi■ e p do Q end and reneat Q end are programs of height h(Q)+1. The execution of exit(i)is intended to cause termination of i enc■ osing whi■ e/repeat_■ 6ops. different from Kosaraju's. Thus the meaning of exit(i)is A■ ■other commands/contro■ structures have the■ r usua■ mean■ ngs. -6- For any nonnegative n, an n― pFOgram ■nteger ■s occurrence of exit(i), i a program conta■ n■ ng neither any 〉 n, nor ha■ t, and an (n+1/2)― program is a prOgFam COntaining no occurrence of exit(i), i of c― program 〉 n. The c■ ass is denoted by RE[c]. We may cons■ der a program to be a f■ owchart. This ■s accomp■ ished by regarding occurrences of ;, then, e■ se and ュo ■n the program as statese We denote by G[Q]the f■ owchart associated with a program Q. For examp■ e, the fo■ ■owing program Q haS the associated f■ owchart G[Q]in Fig。 ■f 3 λe■ se2 ha■ 立 £i 。 , p thenl whi■ e 1。 q 4p4 g ;5 lepeat if r then6 eXIt(1)e■ se7 f fi end end Let Q' be the subprogram repeat ex■ t The r then exit(1)e■ se f fi end state s5 °f G[Q]is the entry state of Ql, state of Qt, and s6 and s7 are s Of is the スノ ー S Q in Q. ■f ` The forma■ defin■ tion of these notions w■ ■■ be c■ ear. Now we define the trace set of a program Q, denOted by L(Q), to be II(G[Q])。 Here the symbo■ of identity action is considered to be the empty word; that is w∈ Σ■. λw = wλ = w for every Thus a f■ owchart is actua■ ■y a finite incomp■ ete automaton w■ th c― moves, but these C― moves are obv■ ous■ y ■nessentia■ onese We say that a f■ owchart G is convertib■ e to a progralll Q if and on■ f■ owchart y if L(G)= G is c― convertib■ e ■f II(Q)。 We a■ so say that a and on■ y if G is convertib■ e to some c― program. Suppose that a f■ owchart G is convertib■ e to a program Q. -7- FoF a Subprogram R of Q, We Say that a state s of G corresponds to the entry state of R if and on■ y if L(く G,s〉 )= L(〈 G[Q], the lf the entry state of R is access■ b■ e the initia■ state of G[Q], there exists such a state s. We entry state of R>). from define s■ m■ ■ar■ y for the exit state and interior states of R. 2.2. COHEN:S RESULT ON STAR EEIGET The star height of a regu■ ar expression over an ■s a nonnegative ■nteger a■ phabet A defined as fo■ ■ows: (1)φ and e■ ements of A have star height O. (2) (α +β )and (α β)have height of star height max(star height of α, star β). (3)α tt has star height (star height of α)+1。 The star height of a regu■ ar event L, denoted by sh(■ ), is the star height of regu■ ar expressions dettoting L. ■east Cons■ der the operation over state graphs of de■ eting one state and re■ ated edges from each max■ of stateso the ■east ■oops ma■ strong■ y connected set The rank of a state graph G, denoted by rank(G), iS number of app■ ications of the operation to break a■ in G. ■ For examp■ e, the rank of state graph in Fig. 2 is l, sinCe the exhaustive search of ways of app■ ications, shown in Fェ g。 5, revea■ s that the 9ptimum way ■s to de■ ete so and s4 at once. Likewise, the rank of state graph in Fig。 4 iS 2. We say that a regu■ ar event L has the fin■ te property if and on■ y if xヽ L /y` L imp■ ies l(x` II)∩ for every pa■ r of words x, y. -8- ■ntersection (y` L)│ < ∞ THEOREM 2.1。 (Cohen [2]) inё omp■ ete SuppOSe that a reduced finite automaton M recognizes a regu■ ar event L. If L has the finite intersection property, sh(■ )= rank(M). We now have the fo■ ■ow■ ng coro■ ■ary. COROLLARY 2。 2. 工f a reduced f■ owchart G is convertib■ e to a program Q and L(Q)has the finite intersectiOn property, then h(Q)〉 rank(G)。 Proof. We first obtain rank(G[Q])〉 sh(IJ(G[Q])) from Egganis resu■ t [4]that the star height of a regu■ ar event does not exceed the rank of any state graph recogn■ z■ ng that event. The coro■ ■ary fo■ ■ows immediate■ y from Theorell1 2.1, the above inequa■ ity and the fact h(Q) Coro■ ■ary 2。 2, app■ ied うe 〉 rank(G[Q]). Q.E.D. under the condition h(Q)= 1, is often to inconvertibi■ ity proofs. A HIERARCHY OF CONTROII STRUCTURES This section ■s the ma■ n part of the paper and is devoted to the path― wise hierarchy ■n Theorem ぅ.8. LEMMA 3el. There exists a f■ owchart which is l― convertib■ e but not 1/2-convertib■ e. -9- Proof. Ilet G=(S,E,s。 )denOte the f■ owchart in Fig. 4。 Then the f■ owchart (S, E-1(sぅ ,gl,SO)│' S4)i' C° nvertib■ e to the fo■ ■owing l― program Ql. f2 ; e■ e■ se se o 2 ., p i 一 f f 一 λ i 2 then 鼈 o ●■ , ” f fつ i一 repeat if p3 then then f2 e■ se Qri上 (1 g2 ; ・ f p2 then f2 e■ se exit(1)塾 墓 end Therefore G is convertib■ e to the fo■ ■owing l― program。 whi■ e pl艶 ギ ql如 e■ se ・ Q1 3 81 f p2 then e■ se Ql ; gl gl墓 墓 end Now assume that G is convertib■ e to a 1/2二 program P. We e state frOm the =ay assume that G[P]contains no lnaccessib■ initia■ state. ■ et Q be a 1/2-program obtained froEI P by changing each subprogram of the form while pl lp ・ 0・ end into p ha■ t end or equ■ va■ ent■ y the program 理 ■f pl then λ else ha■ t fi. Then L(P)= L(Q)since c has exact■ y the program while =巧 One eage Of the form (。 。.,pl,.。 。)and this edge entё rs to the fina■ state s∞ . Hence G iS a■ so convertib■ e to Q. Let Q: be a subprogram of Q Of he■ ght l anc of the fOrm whi■ e r do 。。. end Or 蝉 。 ・・ end. By the construction of Q, the subprogram Q' dOeS not have the fOrm whi■ e pl 12 。c0 9理ユ・ Suppose that Ql is of the form whi■ e p2 1Q R 9ュ ユ. Then -10- the tate s3 0f G corresponds to the entry state of R and the State S2 °°rresponds to the exit state of R, since G has exact■ y one edge of the form(..。 ,巧 ,。 …)・ Thus,since R is a■ oop― free subprogram, in an automaton (S, E-1(s2'p2'S3)l, s。 )there must exist on■ y a finite number of paths from s3 t° S2' a contradictione whi■ e r艶 Likew■ se, Qt Cannot have the form .… 9nd fOr any other r∈ I pl,p2'pァ 弓,ql,■ ,q2'弓 Suppose that Q' is Of the form repeat e.. end. Let s l・ denote the state of e correspOnding to the entry state of Q'。 We may a■ so assume s ≠ s∞ , becauSe if s = s∞ we may change Q' ■nto the program ha■ t by the same reason as the case that we obta■ n Q from Po term■ nation Then, s■ nce ha■ t is the on■ y command to cause of Q', L(Q')= II(G[Ql])= II(<G,s〉 ). Thus from Coro■ ■ary 2。 2 and the fact that x` L(く G,s〉 )≠ yヽ L(く G,s〉 for every pa■ r )imp■ ies (x` L(〈 G,s〉 ))∩ (y` L(く 0,s〉 ))= φ of words x, y, r,nk(く G,s〉 )≦ 1・ But there does not ex■ st such s ■n G, a contradiction。 As readers see in the proof of llemma う。1, Q.E.D. COro■ ■ary 2.2 ows us to do without considering any detai■ ed structure of a■ ■ programs. LEMMA 3.2。 There exists a f■ owchart which is 1/2-convertib■ e but not l― convertib■ e. Proof. The f■ owchart in Fig. 5, denOted by G=(S,E,s。 ), is - 11 - convertib■ e to the fo■ ■owing 1/2-program. whi■ e pl 19 fl , p2 12 f2 ' 崚 wh■ ■e pぅ " fぅ ・ , f p4 then ha■ t Qユ se gぅ fュ end ; ︵ ∠ ・ , 3 製 g5 end Now assume that G is convertib■ e to a l― program Qo We lnay assume that G[Q]contains no inaccessib■ e state from the initia■ state. Let Q: be a subprogram of Q of height l and of the form Whi■ e O・・ dO `.. end or repeat 。。. end。 ■et s be the state of G corresponding to the entry state of Q:, s' be the state of G: corresponding to the ex■ t state of Q:, and W be the set of states of G corresponding to interior states of Q:. We may a■ so assume that s / s∞ and that Q' represents at ■east one ■oop; by the same reason as in the proof of Lemma ぅ。1。 We ttow consider the case s。 ∈W, the case s8∈ W and the remaining Oase, and deduce a contradiction The case s。 ∈W. ■n each case. since s。 ∈W and exit(1)is the on■ y command in Q' tO Cause termination of Q, si must be the fina■ state and hence contro■ to the term■ na■ state. L(Q')= L(〈 G,s〉 ■eav■ ng from Q' means cOntro■ reaching Thus we obta■ n ), and hence from corO■ ■ary2.2 - 12 - rank(く G,s〉 )≦ 1。 But there does not ex■ st such a state s ■n G, a contradiction・ se, the condition S8∈ W eads to , COntradiction. ・ The case s。 The subprogram Q' represents S ・ 8三 生上 `w and exact■ y one ■oop s2 S4 S5 S6 →S2' and hence two states sぅ and Likew■ S7 °f G are ■n (S, E― I(s,a,t) fin■ te W. Thus, s■ nce h(Q・ l a ∈ Σ, t ∈ S}, number of paths from sぅ s。 )=1, in the f■ owchart )there must ex■ st on■ y °r s7 t° S・ a But we cannot find Q.E.D. out such a state s in G, a contradiction. There exiSts a f■ owchart which is IJEMMA う。3。 (1+1/2)― convertib■ e but neither 1/2-convertib■ e nor l― convertib■ e. Fig。 6: The f■ owchart in Fige 6 is seen to be Proof. (1+1/2)― convertib■ e in the same way as in the l― convertibi■ ity proof of Lemtta 3.10 From llerllma 3。 l and 302, the f■ owchart is seen tO be neither 1/2-convertib■e no■ 1-convertib■ e. LEMMA う。4. Q.E.D. There exists a f■ owchart which is both 1/2-convertib■ e and l― convertib■ e but not O― convertib■ e. Proof. The f■ owchart in Fig。 7 iS COnvertib■ e to the 1/2-prograzrl wni■ e p dO・ f q then ha■ t e■ se f fi end, and convertib■ e to the .1-program wh■ ■e p do ■f q then exit(1)9■ se f fi end. The inconvertibi■ ity fo■ ■ows immediate■ y from Kasai [5], -1う ニ since the f■ owchart has a ■00p s。 → sl 'S2'SO and thiS tw9 exits to the fina■ state,(s。 ,F,S4)and (sl,q,S4)・ LEMMA ■oop has Q・ E.D. Given an (n+1/2)― program Q, Where n is a positive う。5・ integer, we can find out an (n+1)― program of trace set L(Q). Proofo We give a required transformation procedure, which inserts some dummy repeat― end structures into Q. exp■ ain First, we the idea by using an examp■ e: Given a (2+1/2)― lprogram whi■ e pi 鯉 p2 ユQ repeat ■f Whi・ e pぅ th食 ユ 1』 e■ p4 then exit(2)e■ sQ fl 』ュ se ■f p5 then ha■ t else exit(1)fi fi end end ; ユ ・ f p6 then exit(1)e■ se f2 £ 蝉 : ; g, the fo■ ■owing ■s a required ぅ―program。 repeat whiユ e pl lp 蝉 repeat if p2 then λe■ se exit(2)fi ; repeat if pぅ 』 p4 th£ ユ Q基 ユ立(3) tL2ユ 主 e■ e■ Se -14- fユ ■f p5 th£ ユ 9基 ユ立(2) e■ end SQ fl . SQ eXit(1)fi fi , end ; ex■ t(3) end ; ・ f p6 then exit(1)e■ se f2 fユ end ; g3 exit(1) end Forma■ ■y, ■et iT姜 ,Tl,T2'・・ ° 'T五 }be the transforIItation system on programs, defined inductive■ y as fo■ ■ows: (1)Tキ (Q)= Fepeat Tl(Q) ; exit(1)end. (2)For every integer i, 1 く i < n, (2.1)if Q is in lF, then Ti(Q)= Q ; (2.2)if Q = exit(j), j く i, then Ti(Q)= 9基 ■立(j) 3 Q = Qxit(j), j ≧ i, then Ti(Q)= exit(j+1) ; (2。 4)if Q = ha■ t, then Ti(Q)= 9基 ュ 主(1) 3 (2。 う)if (2.5)if Q = Rl;R2' then Ti(Q)= Ti(Rl);Ti(R2) ; (2.6)if Q=夏 p then Rl喪Q R2彙 'lhen Ti(Q)= ゴ p then Ti(Rl)e■ Sp Ti(R2)ニ ユ ; (2.7)if Q = repeat R end and i < n, then Ti(Q)= repeat Ti+1(R)9ュ■ ; (2。 8)if Q = repeat R end and i = n, then Ti(Q)= ; exit(n+1)end ; (2.9)if Q = whi■ e p do R end and i く n, then ■229ユ 主 二呈ユ£ュ上 Tl(R)2■ュ Ti(Q)= whi■ e p do Ti+1(R)2ュ ュ ; and (2。 10)if Q = while p do R end and i = n, then Ti(Q)=里 塾 聖 螢 三 p then λe■ se exit(2)墓 Tl(R) -15- ; end ; eXit(n+1) end. The program T姜 (Q)is seen to be a required (n+1)― prOgram. Q.E.De For every n∈ 12,3,4,5,.・ .}, there exists a LEMMA う.6。 f■ owchart which is (n十 ■プ2う ―convertib■ e‐ ut not n― convertib■ e. We first define f■ owcharts Gn=(Sn,En''(1)), Proof. 五∈ 12,3,4,5,0・ ・ }, as fo■ (1)Sn = Is(i) ■ows: l i iS an integer satisfying l ≦ i < 2n+2} U Is(∞ ) }∪ {t(i,j) l i and j are integers satisfying 2n+1 くi く2n+2 and O < j 〈n}. (2)En=1(S(i),p(1),s(2i))11≦ i〈 2n+ll ∪ 1(s(i),p(i5,s(2i+1))11≦ i〈 2n+1} ∪ 1(s(1),f(i),t(i,0))12n11≦ iく U 1(t(i,0),q(i,0),s(∞ )) 1 2n+1 ≦ i 2n■ :│‐ 〈 2■ +21 ∪ 1(t(i,j),q(i,j),s(Li/2n― 計2」 ))12n刊 ≦i 1(t(i,0),q(i,0),t(1,1)) 1 24+1 く i く 2n+21 < ∪ 0 U l(t(i,j),q(i,j),t(i,j+1)) │ ) ) 1 2n+1 ≦ i 〈2n+2} ∪ 1(t(1,n),q(i,n),s(l_i/2」 )) 1 2n+1 ≦ i く 2n+21. We wi■ ■ show that Gn iS (n+1/2)― convertib■ e but not n― convertib■ e. We show the (n+1/2)― conVertibi■ ity by inductive■ y constructing programs Ri, 1 (1)For each integer i, 2n+1 ≦ く i ≦ i 2n+2, as fo■ ■ols: く - 16 - く j〈 n} 2n+1 く i < 2n+2, 。くjく nl UI(t(i,n),q(i,n),s(Li/4」 As an examp■ e, G2 iS Sh° wn in Fig. 8。 2n+2, 2n+2,..et f(i); R.■ = ■f q(I,0)then ha■ t e■ Se ■f q(1,1)then exit(n)e■ se ■f q(i,2)then exit(n-1)e■ se λ fi 3 if q(1,n) then ex■ t(1)e■ se i, 1 < i (2)For each integer Ri=聖 く ゴ p(i)then 蝉 λ fi λ ; λ fi ; fi. 2n+1, .et R2i e■ Se R2i+1 』ユ 饗 ・ The f■ owchart en iS eas■ ■y seen to be convertib■ e to the (n+1/2)― program Rl. Now assume that Gn iS C° nVertib■ e to an n― progran Q. We may assume that G[Q]contains no inaccessib■ e state fron the Let Ql be a subprogram of Q of he■ ght l and of initia■ state. the form whi■ ё ... do ... end or reneat integer i ≧ ■et 2, ・・・ end; and for each Qi be the sma■ ■est subprogram containing Qi■ 1 proper■ y and of the form whi■ e ..e dO ... ёnd or repeat .。 。 end. Ilet m be the greatest number ■ such that Qi eX■ Sts, and ■et S be the set of state, Of en C° rresponding to exit states of Ql, Q2' BeCause of the symmetry in Gn' Ql may be assumed ...,Qmin(m,n)・ to represent a ■oop partition Sn as fo・ ・ passing through s(2n+1)。 Fina■ ■y, we °WS: (1)U_1 = Is(∞ )l. (2)For each integer k, 0 〈 k く n-2, Uk = Is(2k)} ∪ ∪ (2k+1+1)2j<iく (2k+1+2)21,0≦ j≦ Is(i)│ it(i,j) (3)Un_1 = Sn (2k+1+1)2n― ―∪ kE二 予 Uk・ The partition of Sぅ is i■ ・ k n_k} くi 〈(2k+1+2)2n「 卜, 。≦j ≦n}. ustrated in Fig. 9。 Lig。 -17- 9・ We sha■ ■ prove that S∩ Uj ≠φfor eVery j, -1 ≦ j ≦ n_1. Let k = min{i l Ql represents a ■oop passing through s(2・ ), s(2i+1), s(2i+2),..。 , and s(2n+1)}. Then not on■ y for every j∈ lk, k+1,..., n-1}but a■ so for every j∈ 1-1, 0, 1,。 。。,k-1}the State s(23)corre,pOnds to an entry or interior state of Ql, sinCe Ql represents a s(2k)→ ,s(2ktl)→ ..。 ,t(2n+1,1),... .。 . →s(2n+1)→ ■oop of the form t(2n+1,。 ) →t(2n+1,k+1),s(2k), ま nd since(t(2n+1,。 ),q(2ntl,。 ),s(∞ )),(1(2n+1,1),q(2n+1,1),s(1)) , (t(2n+1,2),q(2n+1,2),s(2)),..。 , (t(2n+1,k),q(2n+1,k),s(2k 1)) are edges of Gn Thus S∩ U_1 ≠ φ since Q is an n― program and hence either eXit(1), ex■ t(2),。 .., ex・ t(n_1)Or exit(n)must be used in Ql to represent an edge to the fina■ state, (t(2n+1,。 ),q(2n+1,。 ),s(∞ )). S∩ Uj ≠ φ for eVery j∈ And 10,1,2,。 。。, n 1} since Ql cannOt represent a who■ e part of Uj from Coro■ ■ary 2。 2, h(Ql)三 l and rank((Uj, En∩ Fron the above =LE」 酬 ILEMMA 3。 ■nequa■ ity × Uj), s(23)))2 2. we der■ ve a contradiction, L堕 │IS∩ り =ド │≦ min(町 → ≦ L∝ For every po,itiVe integer n, there exists a 7。 f■ owchart 1≦ (Uj ttΣ which is (n+1)― convertib■ e but not (n+1/2)― convertib■ e. Proofe n∈ 11,2,う We first define f■ owcharts Hn=(sn,En'S(° )), ,。 。0・ }, as fo■ ■ows: -18- L} (1)Sn ls(1) l i iS an integer satisfying O≦ i く 2n+う ∪Is(∞ )}∪ It(1,j) l i and j are integers satisfying 2n+2≦ i<2n+う and。 ≦ j≦ 〒 │ (2)En = 1(s(0),p(0),s(1)),(s(0),p(0),も (∞ ))} +2} U {(s(i),p(1),s(2i))11≦ iく ∪ 1(s(i),p(i),s(2i+1))11≦ iく 2n+2} ∪ 1(s(1),f(1),t(i,0)) 1 2n+2 2■ く i く 2n+う く 一 ヽリ U 1(t(i,j),q(i,j),s( Li/2n― j+2」 l 〈2n+う U l(t(i,0),q(1,0),s(0)) 1 2n+2 < i U 1(t(i,0),q(i,0),t(1,1)12n+2 n}・ } i く 2n+う │ ) 1 2n+2 〈 i く 2n+う , 0く jく nl く < く2■ +う , 0く i く2n+3} < 一 i<2nIラ │. We Wi■ ■ show that 6。 う。 is (n+1)― convertib■ e but not (n+1/2)― convё rtib■ e. We show th6 (n+1)― convertibi■ ity by inductive■ y constructing programs Ri, 1 ≦ i く2n+3, as f。 ..ows: (1)For each integer i, 2n+2 ≦ i く 2n+3, .et f(i)3 Ri = Se ■f q(i,0)then exit(n+1)e■ ■f q(1,1)then ex■ t(n)e■ se if q(i,n) then exit(1 )曇 each integer i, 2 (2)For R. ― ■ lepeat ■f く i p(i) then く λ墓 λ墓 ; ; λ 彙・ 2n+2, .et R2i (3)Let -19- e■ j<nI The f■ owchart Hn is As an examp■ e, Hl is shown in Fig。 10。 to Gn+l in the proof 9f Lemma 一 ∪ 1(t(i,n),q(1,n),s( Li/4」 )) 1 2n+2 ∪ {(t(i,n),q(i,n),s( Li/2」 )) 1 2n+2 sini■ ar 2n+2 ■ 。 U l(t(i,j),q(1,j),t(i,j+1)) se R2i+1 fi end. Rl = whi■ e p(0)19 1』 p(1)thett The f■ owchart Hn 姜 ・ ニュ Ωュユ Rぅ seen to be convertib■ e to the ■y eas■ R2 (n+1)― prOgram Rl。 Now assume that Hn iS C° nvertib■ e to an (n+1/2)― program Q. We may assume that G[Q]cOntains no inaccessib■ e state fron initia■ state. proof of Lemma Ilet Qi, m and S be the same as defined in the う ・ 6. Because of the symmetry in Hn, Ql may be assumed tO represent a partition Sn (1)U。 the s fo■ ■oop passing through s(2n+2)e And we °WS: ・ =Is(0),S(1)}u{S(1)│ラ ×2j≦ iく 4× 2j,0≦ j≦ u it(i,j)13× 2n+1≦ i<4× 2n+1,。 ≦ j≦ nl. (2)For eaCh integer k, 1 ≦ k n+11 く n, Uk=Is(2k)} ∪ Is(1) │(2k+1+1)2j ≦ i く (2k+1+2)2j, 0≦ j ≦ n_k+11 k+1, k+1 く i く (2k+1+2)2n― ∪ it(i,j) │(2k+1+1)2n― 0≦ j≦ (3) Un=Sn-lS(∞ )}… uk曇 n}・ l Uk・ る The partition of S2 is i■ ■ustrated in Fig。 11。 Then we obta■ n S∩ Uj / φ ■n fOr every j∈ the same manner aS Σ j=8 1 く 一 n+l = ■n .,n} 10,1,2,。 。 the proof of ■emma う。6。 Ujl≦ 應│≦ Σ j=8り ∩ min(m,n)≦ n, Q.E.D。 contradiction. From above seven Thus ■emmas we obtain the fo■ ■owing resu■ t。 - 20 - THEOREM う。8. 型LRコ 2]―⊇ where each IL(Rコ i◆ ■ine [2]pw RE[2+1/2]pw from RE[c]to RE[d]means [c])ン L(RE[d]) e.{L(Q) I Q (2)L(RE[11)― ∈ RE[c]}ァ lL(Q) I Q CRE[d]│. ■(RE[1/21)≠ φ, L(RE[1/2])― ■(RE[1])/ IL(RE[1+1/2])― (L(RE[1/2]) UL(RE[1]))/ (■ (RE[1/2])∩ ■(RE[1]))― II(RE[0])/ Φ, φ, φ. Incidenta■ ■y, Peterson et a■ .[8]shows that L(RE[∞ ])= {L(G) I G is a f■ owchart}. This can be sharpeneo by COns■ der■ ng ranks of f■ owcharts. The proof is simi■ ar to the proof of Eggan's resu■ t [4]that giヤ e五 an incomp■ ete automaton M, we can find out a regu■ ar expression of star height rank(M)and of THEOREM う。9。 ■anguage L(M). 19t G=(S,E,s。 )be a f■ owchart. If rank(G)= 0, then we can construct a O― program of trace set L(G)by using on■ y semico■ on and if― then― e■ se=fi structures; if rank(G)= 1, then L(G)∈ L(RE[1/2]) ∩ L(RE[1]); and if rank(G)〉 1, then L(G)∈ L(RE[(rank(G)-1)+1/2]). Proof. An i― section is defined to be a maxュ ma■ StrOng■ y connected sё t T of states such that iank((S,(T× ΣXT)∩ E,s。 ))= i. ■et n(i,G)be the number of i― sections in G, and k(G)= Σ 主 =Tank(G)n(i,G)IS li. -21- ■et 一 ・ The proof proceeds Basis, k(G)= ■oop, by induction on k(G). 0. Since rank(G)=O and hence G contains we can eas■ ■y construct a requ■ red O― program. Induction step, k(G)〉 0。 Let K be the c■ ass of ■et rank(G)― sections, and for any T∈ K s(T)be a state in T such that rank((S, ((T-ls(T)1)XΣ X(T-ls(■ )l))∩ E, s。 ))= rank(G)-1。 We gradua■ ■y construct a requ■ red program. In the first step we construct a required program Q[T]for each f■ owchart くG,s(T)>, Tこ K. EXIT[T]={s∈ S― ■ ■et │(T× Σ×Isl)∩ E≠ φ}, ■et and G[T]= (S,(E ― XS) Is(T)})× Σ (EXIT[T]U U{(t,f(t),s(T)) l t ∈EXttT[T]}, s。 ) where f(t)'s are new symbo■ s in lF. The f■ owchart G[T]is ustrated in Fig。 12. i■ ■ (1)The case that rank(G)_>_1_and_(s(■ ),g,Sl )∈ From the inductive hypothesis and and g∈ lF. E` fbr some ,1三 査 「 ■ettma 305, We can construct (rank(G)-1)― prOgr,五 s Q[sl]Of trace s9t L(〈 G[T],sl〉 )・ From the inductive hypothesis and the fact that f6r any t∈ EXttT[T]the f■ f■ owchart a■ so o■ chart くG,t> can be reduced intO a O' satisfying k(G')〈 k(G), for each t∈ EXttT[T]we can construct a ((rank(G)-1)+1/2)― program R[t]of trace set L(〈 G,t〉 ). 里 鍵 Then g;Q[Sl]1(R[t];肇 螢 )/1(t)}t∈ EXIT[T]型 is a required ((rank(G)-1)+1/2)― program Of trace set L(く G,s(T)〉 ) where Q[sl]{(R[t] ; ha■ t)/f(t)lt∈ EXttT[T]iS a program obtained frOm Q[Sl]by rep■ acing each occurrence of f(t), t∈ EXIT[T], by - 22 - 平 亜互 the program R[t];ha■ t. (2)The case that rank(G)〉 l and (S(■ ),p,S 1 for some sl, s2∈ S and p∈ P. )∈ E We can construct a requ■ red program in the same manner as in the caSe (1). In fact, repeat if p then Q[sl]1(RIt];ha■ t)/f(t)}t∈ EXttT[T] else Q[S2]1(R[t];ha■ t)/f(t)}t∈ EXIT[T]墓 end is a required program′ where Q[S2] iS a (rank(G)-1)― prOgram of trace set L(〈 G[T],s2〉 )° (3) The caseithat rank(G)= l and (S(■ ),g,Sl )∈ E for some s and g∈ F. O― program From the ■nductive hypothes■ s we can construct ∈S a Q[Sl]Of trace set II(く G[T],sl〉 )by using on■ y semico■ on and if― then― e■ se― fi structures. From the inductive hypothesis and the same fact as in the case (1), for each t∈ l― EXIT[T]we can a■ so construct a 1/2-program P[t]and a program R[t]oF trace set L(く G,t〉 ). Then ■S 1 ■epeat g ; Q[sl]1(P[t];ha■ t)/f(t)}t∈ EXttT[T]9三 コ a required 1/2-program of trace set IL(〈 G,s(■ )〉 ); and ■S epeat g;Q[、 ]1は [t];彙 (1))/f(t)}t∈ EttTETi頭 ■ a required l― program of trace set L(く G,s(T)>)since Q[Sl]haS no e/rep£ at― ■oop. wh■ ■ (4) The case that rank(G)= l and (S(■ ),p,Sl), (s(T),D,s2)∈ E for some sl, s2∈ S and p∈ IP_・ We can construct requ■ red programs in the same manner as in the case (う )。 In the second step we construct a required program for G。 Let H=(Su itl,(E-ls(T)IT∈ ∪ K}× Σ×S) 1(s(T),g(T),t)I T∈ - 2う ― KI, s。 ) where t is a new state and g(T)'s are nё w symbo■ s f■ owchart H is i■ ■ustrated in Fig。 1ぅ O in l「 . The From inductive hypothesis and the fact that H can be reduced into a f■ owchart Hi satisfying k(Hl) く k(H), we can cOnstruct a ((rank(G)-2)+1/2)― program R of trace set IL(H)。 Then RIQ[T]/g(■ )IT∈ K is a required ((rank(G).1)+1/2)― program Of trace set L(G)where Q[T]ts are ((rank(G)-1)+1/2)― programs obtained in the first Q・ EoD. step. ACKNOWLEDGMENTS I w■ sh ■ntroduc■ ng to thank Professor Teruyasu Nishizawa lor me to the area, and the referee and Professor Kojiro Kobayashi for reading carefu■ ■y the manuscript. They a■ so gave me many va■ uab■ e suggestions/advices. REFERENCES Ro Se Cohen and 1。 」。 A. Brzozowski, Genera■ properties of star height of regu■ ar events, 」。 Computo System Sci。 4 (1970), 260-280。 Ro Se Cohen, Star height of certain fami■ ies of regu■ ar 2。 events, う. 」。 Comput. System Sci。 4 (1970), 281-297. Eo W. Dijkstra, GoTo statё ment considered harmfu■ , comm. ACM ll (1968), 147-148. … 24 - 4・ R. C. Eggan, Transition graphs ana the star he■ ght of regu■ ar events, Michigan Math. 5・ T・ 」。10 (196う ), 385-397. Kasa■ , Trans■ atabi■ ity of f■ owcharts_■ nto whi■ e prograns, 」. Comput. System Sci。 9 (1974), 177-195。 6. S. R. Kosaraju, Ana■ ysis of structured programs, 」. Comput. System Sci。 9 (1974), 232-255・ 7。 H. F. Iledgard and M. Marcotty, A gene■ ogy of contro■ structures, Comme ACM 18 (1975), 629-6う 9. 8. W. Wo Peterson, T. Kasami and N. Tokura, On the capabi■ ities of Whi■ e, Repeat, and Exit statements, Comm. ACM 16 (197う 50う -512. -25- ), Descriptive ■egends for figures. Fig.■ . An examp■ e of G[Q]. Fig.2. A State graph of rank Fig。 . An exp■ anation of the Fact that the rank ttf the state 3. graph in Fig。 2 is Fig。 ■ ■. A f■ ow9hart of rank 2 in Lemma3.■ 4. . Fig.5。 A f■ owchart in Lemma3.2. Fig.6. A f■ owchart in Lemma3.3. Fig。 7. A f■ owchart in Lemma3.4. Fig。 8. A f■ owchart G2 in the proof of Lemma3.6` Fig。 9. Fig。 ■0. Fig.■ ■. The partition oF S3 in the proof of Lemma3.6. A f■ owchart H. in the proof of Lemma3.7. The partition of S2 in the proof of Lemma3.7. Fig.■ 2. A f■ oWChart Fig。 ■3. G[T]in the proof of Theorem3.9. A f■ owchart H in the proof of Theorem3.9. - 216 - Fig。 ■ . - 271 - Fig.2. - 28 _ Fig.3. - 29′ ― Fig.4. - 130- Fig.5. - 31 - Fig。 6。 -32- Fig。 7. -33- く2) Fig.8. 34- 銅 ← 0 ≧ … 個 動 0 く1) 0 金 0 し ) ■ 弊 〇 喝 Fig.9. -35- qC8J) 一 一 0 ゛ 蜂 qG′ Fig.10. - 36- U0 Fig:1■ -3_7- . f(tt) tt n EXIT[T] t2 n EXIT[T] Fig。 ■2. -38- Fig。 13 - 39-
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