ON A FEW CLIQUE−GRAPH EQUATIONS
Iwao SATO (Received Nbvember 10, 1978〕 Abstract We shall give the solutions of graph equations C rL rG))_C」C(MrG〃=G and cr夕rGノ)=G. 1 Definitions AIl graphs which we treat in the paper are finite mdirected without nrultipule edges a皿d loops・ We denote by VrGノ。ErG) the vertex set and the edge set of a graph G」 respectively. 〃rO/ ofσ is an intersection graph St(F/3where 匹{{・}1・∈γrω}U醐・Definiti・ns・・t・presented・here, may b・f・md血[3]. 2Theorems th・・r㎝1th…Z・ti・n 9・qph G・f・9・uph・〈櫛鋤c伍rG〃−G i・the・鳩 graphs whi・h・atisfy f・ Z Z・・7ing』θθ・・nditi・η8ω。r2ノαnd r3方 ωne deσree・了⑳ery ve?tex・了G isαカZeast tU。. 「2/be・iY P・ir・かω・励・ηgZθ8・アσ・is e・ig・臨ゴ・仇力. 「3/Ev・rg tni・ngZ・・f G ha・e・a・tZy…vertec・f d・ev・・鋤. f「・。f〔・→SupP・se thatθis a g・aph satisfying th・three c。nditi。n,. Ifσh・・n・t any t・i鋤91・, th・n the eq・・ti・・CrLrG))−G bec㎝・・0・rG).G. ,ince in thi・ca・e L俳・rの・Acc・・di・gly,・(L‘σ〃=G h・1d・if飢d・nly if・v。ry ve「tex・f C i・・fd・g・ee at least t・。 by(【1],S・t・4). W・m・y・・n・ider i。 th。 f°11。・i㎎・ca・e wh・nσ・・nt・ins t・iang1・・. C…truct th・1i・・g・aph.LrGノ。f σ・アen the「e c°「「es恥t。・v・ry・・i孤・…」rゴ・・・・・…随・一・ry
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Ftlcesり声・…・s) excePt f・r v・・tice・・」rゴ……・・)・f d・gree加・・n・h・鵬e。1「1瀞跳;ll・熾蹴さ;・。ll:1嘘elllse竺1N認1認aa「e
mapPil19 φ of VrG) onto VrC(L rGノ)) as follows:恥羅i;i篇蕊illl:欝i;:d:蕊;膓’一・1・
(・〕th・ma即ingφi・abij・cti・n・f VrGノ・nt。 VrOrLrσノ〃.1鷲・ご惣2二1㌶ぷ跳罐。㌔
3132 1..SATO .1 adj acent in o rz}rσノ). (・)L・tb・th’∂k・・〆k・ゴ」・・…・・ノb…tvertices°f deg・ee tw°°f t「i孤91es
聯蹴ム,a霊慧’nG⇔X晒侮ソ͡ 〉φ「桝d
Fr㎝(α〕,(β〕and(Y〕,we know thatφis an iscmK)rphisin of G o直toσr乙rG〃. Therefore, the graphσ satisfying three conditions (1), 〔2〕 al且 〔3〕’satisfies the equation C CLCG)▲−C・. 、 ←〉) Let G sa土isfy the equation OrZ}CG)ノ=G and conaition (2),except・for (1) or (3〕. If all vertices of a triangle of G have degrees greater than ltwo, th・・1@we h・v・0.r五rG〃−G⊃K4(Fig・1)・Tl・i・c。就radi・t・t・.(2)・}le・ce・at m°st σ 、 、 @ @ @ @ @ @ @\ @ @ @ @_、一
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C(T(G)) (Fi・9.1) two vertices of evefy triangle are of degree・greater than two・ Then, a cliqUO 1ζof 1}(G) ’is one of the following(c.f.[4]〕: . (・)K−Lr・、,ρr。ノ==K,r。ノ・砲・・e eith…醐G”is°n a t「i㎎1e鋤ρf∂ノ 華..・rびis・n n・tri孤91e飢dρω≧2・ 〔2〕醐・ゴ)’「ゴー1・…・・)・油・re『{ち・…・T.}is a set°f t「iangles°f G・’ Now, we consider the follσwipg mappヰ㎎ψfr㎝V(C(LでG川toγrω:‡f醐ち。ρωノ・κρω』th㎝ψω⇒・ ’ ..
if X≒Z}r㌘J r右2⊃...3r)5 thenψ(K)_.v, where∂ is a vertex of degree七wo of 』9 at麺91e㌘ゴ・ It.is clear that’ψis a inj ecti㎝. Thus, the nmi)er of clique§ of Z}rσノ is at most lγr釧. Consequent1γ, if eitherσhas.avertex of degree』㎝e, or蜘o vertices・of a triangle of G are of degree two, then we obtain an inequality IγrO伍rω.川く1』γ‘ω1. T厄s contradictS the assu叩ti㎝Or砲〃−C. Next, 1et G do not satisfy the condition (2〕.and contaill an induced sUb− graph K4−x whi・h・。ns.ist・・f加・tri・ngles・h・ving・・n・ve「t朕mC㎝∞n・Ktl♪ Ktn
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k1 2 k (Fig.2) If G satisfies the eqμとtion O(LrG〃=G. then there is a subgraph冴of G・・d血t血it・1桓・9r・ph・LrH). c・卿1・t・・ubg・、p㎏Xω.〆2㌔f乃㈲㎞。
・・rtex・in・ctm・・舳・each・f t㎞ee d卿1・t・鋤・gra旗・叉ω。・i,b。 th孤itself, ・eSP・・tiV・1y」魂・卿1・t・ ・ubgr・ph、 K「3ノ.K「4) 。f L(E) h。。 a v。rt,)x・in。⑩㎝ ・i・head・・f・K’(’)・nd・K「2ノ,・・SPec・iv・・y(Fig.2). Ac。。。di。g,。 ICrausz’s Theorem,there may be such a graph. But・these °ccu「s伽・t・i・㎎1es<{vユ・・2・・3}・・く{・、・v4・・5}・・i・五伽血i・h罐裁篇竺:li。、㌻e。:蒜11霊el。竪ll㌶e:。w纂
・T㎞s・G㎜・t・・砲㎞…bg・・ph K4・Since・CrLrσ〃−C.鋤G・・就・mS・・ub二.
graph K4・G・mu・t・・nt・in crLrK4」ノ〔Fig・3)・ K4 L(K4) C(L(K4)) (Fig.3) H°weve「・0‘五「K4”c°nt・ins K4・紐d・・C・・武・in・2K4・S血丘1・・1y・σr五rG〃−C :c°ntains 2C「L(K4”=4㌦・C。nt迦ing this p・㏄ess,σb㏄㎝・・飢血・finit・9r・ph, but this is a c㎝tTadiction.}lence, if G does not satisfy condition(2), th㎝θd・es n・t・ati・fy.th・equati・n CrL‘θ〃=G./34
1.SATO
Theor㎝2tZVIe soZution 9i・aph G of graph equαtion c rMrσ〃=σのθthe o殉 gmaPhe whiσh oontainπo’triangZes. Proof. 1・et G be a gTaph obtained by adding to G new p vertices v撒ri=23…・《・一垣ges・砲・r・・−1・・釧輌ω一{・、・・・…,}・…n…あi・
is㎝oτphic to丑f(Gノ ([2],Th 1)・ Hence・the equation CθM(G〃=G considered may be rewrittell as O r五r(;+))=G. 〔1)L・tθ・・nt・桓n・t・i飢91・・. Dra疏g th・1in・g・・.ph・Lrめ・fσチ。 ・i㏄・σ・・nt・in・n・t・i孤91・・, in thi・case e・ch・liqy・・f五rめi・th・1in・ graph of the sゆgraph induced by some vertex of G and its neighboエho(K【in G (Fig.4)・? ↑ ↑ ♀ ♀ 9
‘ ‘ t ω’ 1 」 1 ∂ 1 ‘ ‘ o−一一一 ー←‘1Φ G+ ‘1−10 一一一,r 《》一6 6
L(G+)(heavy lines) 一◎ 儂。b::1:㍑芸&「e) ・ (Fig.4)th・・, th・n迦b・f・f・1iqu…f五rめi・equa1 t・17r釧.
Each vert・)x・f五(めb・1・㎎・t・at m・st tw・cliqu・・〔[3],Tl・ 8.4). lh・蒜’s㌶鑑。’;,蒜三,lxえt霊。麟(:,監;瀞∫tc;。’n
thi・ca・e, by n・glecti・g・uni・1iq・・1・・rtice・,ガヱb㏄㎝。、 th。叩eratエ㎝・, and then, G is obtained. (2〕S”pP°se.thatθc°ntains t「iangl…Al1 three v・垣ce・・〆ゴ・vk°f㌫三e。1;㌶{認>tlil,IC、hceale;:::t°1。:t,li三:.曲「㌘:、篇lll・
rめ〃li.・.・〔〃(G〃−C〔LCめ),‘G./th…㎝.3 th…Z・ti・n 9卿h G・丁θ・鋤・(1・・iti・n crTrG〃−G鯉・殉 ,totaZZy〈iisconneeカed gra穀hs. Proof・If G is a totally disco皿ected graph, then it is clear that O「a’「G)」−T(・[[heref・・e・1・t G b・an・nt・ivi・1・・m・ct・d・9r・ph, and。x、㎜。 th。 cliques of its tota]:graph. e3 e e 3 e2={Vl,v3} 2 5 θ5 4 4 り (Fig.5)