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Mathematical Problems of Computer Science 40, 31{33, 2013.

Simple P r oofs of T wo Dir ac-type T heor ems I nvolving

Connectivity

Ca r le n M. Mo s e s ya n , Mh e r Zh . N iko g h o s ya n a n d Zh o r a G. N iko g h o s ya n

Faculty of Mathematics and Informatics
Kh. Abovyan Armenian State University

Institute for Informatics and Automation Problems of NAS RA

e-mail: mosesyan@list.ru, zhora@ipia.sci.am

Abstract

In 1981, the third author proved that each 2-connected graph G with ± ¸ (n+·)=3 is
hamiltonian and each 3-connected graph contains a cycle of length at least minfn; 3± ¡
·g, where n denotes the order, ± - the minimum degree and · - the connectivity of G.
Short proofs of these two results were given by HÄaggkvist and Yamashita, respectively,
occupying more than three pages for actual proofs altogether. Here we give much
simpler and shorter proofs actually occupying the two-thirds of a page.

Keywords: Minimum degree, Connectivity, Circumference.

L e t G b e a ¯ n it e u n d ir e c t e d g r a p h wit h o u t lo o p s o r m u lt ip le e d g e s . A g o o d r e fe r e n c e fo r a n y
u n d e ¯ n e d t e r m s is [1 ]. W e r e s e r ve n, ±, · a n d c t o d e n o t e t h e o r d e r ( t h e n u m b e r o f ve r t ic e s ) ,
m in im u m d e g r e e , c o n n e c t ivit y a n d c ir c u m fe r e n c e ( t h e le n g t h o f a lo n g e s t c yc le ) o f G. Th e
ve r t ic e s a n d e d g e s c a n b e in t e r p r e t e d a s s im p le c yc le s o f le n g t h s 1 a n d 2 , r e s p e c t ive ly. A
c yc le C in G is c a lle d a H a m ilt o n c yc le if jCj = n a n d is c a lle d a d o m in a t in g if GnC is
e d g e le s s .

In 1 9 5 2 , D ir a c [2 ] p r o ve d t h a t e a c h g r a p h wit h ± ¸ n= 2 is h a m ilt o n ia n ( i.e . h a s a H a m ilt o n
c yc le ) a n d in e a c h 2 -c o n n e c t e d g r a p h , c ¸ m in fn; 2 ±g.

In 1 9 8 1 , t h e t h ir d a u t h o r [5 ] wa s a b le t o o b t a in t wo a n a lo g o u s D ir a c -t yp e r e s u lt s in vo lvin g
c o n n e c t ivit y ·.

T heor em 1 [5]: E very 2-connected graph with ± ¸ ( n + ·) =3 is hamiltonian.
T heor em 2 [5]: In every 3-connected graph, c ¸ m in fn; 3 ± ¡ ·g.
Th e o r ig in a l p r o o fs [5 ] o f Th e o r e m s 1 a n d 2 a r e s o m e wh a t le n g t h y a n d c o m p lic a t e d .

S h o r t p r o o fs o f t h e s e t wo r e s u lt s we r e g ive n b y H Äa g g kvis t [3 ] a n d Y a m a s h it a [7 ], r e s p e c t ive ly,
o c c u p yin g m o r e t h a n t h r e e p a g e s fo r a c t u a l p r o o fs a lt o g e t h e r .

In t h is n o t e we p r e s e n t m u c h s im p le r a n d s h o r t e r p r o o fs o f t h e o r e m s 1 a n d 2 ( a c t u a l p r o o fs
o n a b o u t 2 / 3 p a g e ) b a s e d m a in ly o n s t a n d a r d a r g u m e n t s a n d t h e fo llo win g t wo t h e o r e m s .

T heor em 3 [4]: L et G be a 2-connected graph with ± ¸ ( n + 2 ) =3 . Then every longest
cycle in G is a dominating cycle.

T heor em 4 [6]: L et G be a 3-connected graph. Then either c ¸ 3 ± ¡ 3 or every longest
cycle in G is a dominating cycle.

Th e s e t o f ve r t ic e s o f a g r a p h G is d e n o t e d b y V ( G ) a n d t h e s e t o f e d g e s - b y E ( G) .
Fo r S a s u b s e t o f V ( G) , we d e n o t e b y GnS t h e m a xim u m s u b g r a p h o f G wit h ve r t e x s e t

3 1



3 2 Simple Proofs of Two Dirac-type Theorems Involving Connectivity

V ( G ) nS. Fo r a s u b g r a p h H o f G we u s e GnH s h o r t fo r GnV ( H ) . W e d e n o t e b y N ( x) t h e
n e ig h b o r h o o d o f a ve r t e x x in a g r a p h G. W e wr it e a c yc le C o f G wit h a g ive n o r ie n t a t io n

b y
¡!
C . Fo r x; y 2 V ( C ) , we d e n o t e b y x¡!C y t h e s u b p a t h o f C in t h e c h o s e n d ir e c t io n fr o m

x t o y. Fo r x 2 V ( C ) , we d e n o t e t h e s u c c e s s o r o f x o n ¡!C b y x+ a n d t h e p r e d e c e s s o r b y x¡.
Fo r X ½ V ( C ) , we d e ¯ n e X + = fx+jx 2 Xg.

Lemma 1: L et G be a 2-connected graph and S - a minimum cut-set in G. If every
longest cycle in G is a dominating cycle, then either c ¸ 3 ± ¡ · + 1 or there exists a longest
cycle C with S µ V ( C ) .

P r oof: Ch o o s e a lo n g e s t c yc le C in G s u c h t h a t jV ( C ) \ Sj is a s g r e a t a s p o s s ib le .
A s s u m e t h e c o n ve r s e , t h a t is S 6µ V ( C ) a n d x 2 SnV ( C ) . S in c e C is d o m in a t in g , N ( x ) µ
V ( C ) . L e t »1; :::; »t b e t h e e le m e n t s o f N ( x ) , o c c u r r in g o n

¡!
C in a c o n s e c u t ive o r d e r . P u t

M1 = f»ijV ( »+i
¡!
C »¡i+1 ) \S 6= ;g a n d M2 = N ( x ) nM1. S in c e x 2 S, we h a ve jM1j · ·¡ 1 a n d

jM2j = jN ( x ) j ¡ jM1j ¸ ± ¡ · + 1 . Fu r t h e r , s in c e C is e xt r e m e a n d jV ( C ) \ Sj is m a xim u m ,
N ( x ) \ N + ( x) \ M ++2 = ; a n d c ¸ jN ( x ) j + jN + ( x ) j + jM ++2 j = 2 jN ( x ) j + jM2j ¸ 3 ± ¡ · + 1 .

P r oof of T heor em 2: L e t G b e a 3 -c o n n e c t e d g r a p h , S b e a m in im u m c u t -s e t in G
a n d le t H1; :::; Hh b e t h e c o n n e c t e d c o m p o n e n t s o f GnS. Th e r e s u lt h o ld s im m e d ia t e ly if
c ¸ 3 ± ¡ 3 , s in c e 3 ± ¡ 3 ¸ 3 ± ¡ ·. Ot h e r wis e , b y Th e o r e m 4 , e ve r y lo n g e s t c yc le in G is a
d o m in a t in g c yc le . If S 6µ V ( C ) fo r e a c h lo n g e s t c yc le C t h e n b y L e m m a 1 , c ¸ 3 ± ¡ · + 1 .
L e t C b e a lo n g e s t c yc le wit h S µ V ( C ) . If V ( GnC ) = ; t h e n jCj = n a n d we a r e d o n e . L e t
x 2 V ( GnC ) . A s s u m e w.l.o .g . t h a t x 2 V ( H1 ) . P u t Y1 = N ( x ) [ N + ( x) . Cle a r ly jY1j ¸ 2 ±
a n d it r e m a in s t o ¯ n d a s u b s e t Y2 in V ( C ) s u c h t h a t Y1\Y2 = ; a n d jY2j ¸ ±¡·. A b b r e via t e ,
V1 = V ( H1 ) [ S. S u p p o s e ¯ r s t t h a t Y1 µ V1. If V ( H2 ) µ V ( C ) , t h e n t a ke Y2 = V ( H2 )
s in c e jV ( H2 ) j ¸ ± ¡ · + 1 . Ot h e r wis e , t h e r e e xis t y 2 V ( H2nC ) wit h N ( y ) µ V ( C ) ( s in c e
C is d o m in a t in g ) a n d we c a n t a ke Y2 = N ( y ) nS. N o w le t Y1 6µ V1. A s s u m e w.l.o .g . t h a t
Y1 \ V ( H2 ) 6= ;. S in c e N ( x ) µ V1, we c a n c h o o s e z 2 N + ( x ) \ V ( H2 ) . If N ( z ) µ V ( C ) , t h e n
t a ke Y2 = N ( z ) nS, s in c e N + ( x) is a n in d e p e n d e n t s e t o f ve r t ic e s ( b y s t a n d a r d a r g u m e n t s ) a n d
t h e r e fo r e , N ( z ) \ N + ( x ) = ;. Ot h e r wis e , c h o o s e w 2 N ( z ) nV ( C ) . Cle a r ly N ( w ) µ V ( C ) ,
w 2 V ( H2 ) a n d N ( w ) \ N + ( x) = fzg. Th e n b y t a kin g Y2 = ( N ( w ) nfzg ) n ( Snfz¡g ) we
c o m p le t e t h e p r o o f.

P r oof of T heor em 1: A s s u m e t h e c o n ve r s e , t h a t is G is a n o n -h a m ilt o n ia n 2 -c o n n e c t e d
g r a p h wit h ± ¸ ( n+·) =3 . L e t S b e a m in im u m c u t -s e t in G. S in c e ± ¸ ( n+·) =3 ¸ ( n+2 ) = 3 ,
b y Th e o r e m 3 , e ve r y lo n g e s t c yc le in G is a d o m in a t in g c yc le . Fu r t h e r , s in c e c · n · 3 ± ¡·,
b y L e m m a 1 , G c o n t a in s a lo n g e s t c yc le C wit h S µ V ( C ) . A s in t h e p r o o f o f Th e o r e m 2 ,
c ¸ 3 ± ¡ ·, c o n t r a d ic t in g t h e fa c t t h a t ± ¸ ( n + ·) / 3 .

Refer ences

[1 ] J.A . B o n d y a n d U .S .R . Mu r t y, Graph Theory with Applications, Ma c m illa n , L o n d o n a n d
E ls e vie r , N e w Y o r k, 1 9 7 6 .

[2 ] G. A . D ir a c , \ S o m e t h e o r e m s o n a b s t r a c t g r a p h s " , P roc. L ondon, M ath. Soc., vo l. 2 , p p .
6 9 { 8 1 , 1 9 5 2 .

[3 ] R . H Äa g g kvis t a n d G.G. N ic o g h o s s ia n , \ A r e m a r k o n h a m ilt o n ia n c yc le s " , J ournal Com-
bin. Theory, S e r . B 3 0 , p p . 1 1 8 -1 2 0 , 1 9 8 1 .

[4 ] C. S t . J. A . N a s h -W illia m s , Studies in P ure M athematics, ( E d g e -d is jo in t h a m ilt o n ia n
c yc le s in g r a p h s wit h ve r t ic e s o f la r g e va le n c y, p p . 1 5 7 { 1 8 3 ) , in : L . Mir s ky ( E d ) , A c a d e m ic
P r e s s , S a n D ie g o , L o n d o n , 1 9 7 1 .



C. Mosesyan, M. Nikoghosyan, Zh. Nikoghosyan 3 3

[5 ] Zh . G. N iko g h o s ya n , \ On m a xim a l c yc le o f a g r a p h " , D AN Arm.SSR , ( in R u s s ia n ) , vo l.
L X X II, n o . 2 , p p . 8 2 -8 7 , 1 9 8 1 .

[6 ] H .-J. V o s s a n d C. Zu lu a g a , \ Ma xim a le g e r a d e u n d u n g e r a d e K r e is e in Gr a p h e n I" , W iss.
Z. Tech. Hochschule Ilmenau, vo l. 2 3 , p p . 5 7 -7 0 , 1 9 7 7 . ( M ath. Soc., vo l. 2 p p . 6 9 { 8 1 ,
1 9 5 2 .)

[7 ] T. Y a m a s h it a , \ A d e g r e e s u m c o n d it io n fo r lo n g e s t c yc le s in 3 -c o n n e c t e d g r a p h s " , J ournal
of Graph Theory, vo l. 5 4 , n o . 4 , p p . 2 7 7 -2 8 3 , 2 0 0 7 .

Submitted 05.09.2013, accepted 17.10.2013.

γå³Ïóí³ÍáõÃÛ³Ý å³ñ³Ù»ïñáí ¸Çñ³ÏÛ³Ý ï»ëùÇ
»ñÏáõ ûáñ»ÙÝ»ñÇ å³ñ½ ³å³óáõÛóÝ»ñ

Î. Øáë»ëÛ³Ý, Ø. ÜÇÏáÕáëÛ³Ý ¨ Ä. ÜÇÏáÕáëÛ³Ý

²Ù÷á÷áõÙ

ºññáñ¹ Ñ»ÕÇݳÏÁ 1981-ÇÝ ³å³óáõó»É ¿, áñ Ï³Ù³Û³Ï³Ý 2-ϳå³Ïóí³Í ·ñ³ý ± ¸
( n+·) =3 å³ÛÙ³ÝÇ ¹»åùáõ٠ѳÙÇÉïáÝÛ³Ý ¿, ÇëÏ Ï³Ù³Û³Ï³Ý 3-ϳå³Ïóí³Í ·ñ³ý áõÝÇ
³éÝí³½Ý m in fn; 3 ± ¡·g »ñϳñáõÃÛ³Ý óÇÏÉ, áñï»Õ n-Á ·ñ³ýÇ ·³·³ÃÝ»ñÇ ù³Ý³ÏÝ ¿, ± -
Ý‘ Ýí³½³·áõÛÝ ³ëïÇ׳ÝÁ, ÇëÏ ·-Ý` ϳå³Ïóí³ÍáõÃÛáõÝÁ: г·íÇëÃÁ ¨ Ú³Ù³ßÇï³Ý,
ѳٳå³ï³ë˳ݳµ³ñ, ½·³ÉÇáñ»Ý Ïñ׳ï»É »Ý ³Ûë »ñÏáõ ûáñ»ÙÝ»ñÇ Ý³ËݳϳÝ
³å³óáõÛóÝ»ñÁ‘ »ñ»ù ¿çÇ ë³ÑÙ³ÝÝ»ñáõÙ: Ü»ñϳ ³ß˳ï³ÝùáõÙ ÁݹѳÝáõñÁ 2/3 ¿çÇ
ë³ÑÙ³ÝÝ»ñáõÙ Ý»ñϳ۳óíáõÙ »Ý ß³ï ³í»ÉÇ Ï³ñ× ¨ å³ñ½ ³å³óáõÛóÝ»ñ:

Ïðîñòûå äîêàçàòåëüñòâà äâóõ òåîðåì Äèðàêñêîãî òèïà
ñ ïàðàìåòðîì ñâÿçíîñòè

Ê. Ìîñåñÿí, Ì. Íèêîãîñÿí è Æ. Íèêîãîñÿí

Àííîòàöèÿ

Òðåòèé àâòîð â 1981 ãîäó äîêàçàë, ÷òî êàæäûé 2-ñâÿçíûé ãðàô ïðè óñëîâèè ± ¸
( n+·) =3 ãàìèëüòîíîâ, à êàæäûé 3-ñâÿçíûé ãðàô ëèáî ãàìèëüòîíîâ ëèáî ñîäåðæèò
öèêë äëèíû ïî ìåíüøåé ìåðå 3 ± ¡ k , ãäå n îáîçíà÷àåò ÷èñëî âåðøèí ãðàôà,
±- ìèíèìàëüíàÿ ñòåïåíü è k - ñâÿçíîñòü. Õàãâèñò è ßìàøèòà, ñîîòâåòñòâåííî,
çíà÷èòåëüíî ñîêðàòèëè îðèãèíàëüíûå äîêàçàòåëüñòâà ýòèõ òåîðåì â ðàìêàõ òðåõ
ñòðàíèö. Â íàñòîÿøåé ñòàòüå â ðàìêàõ 2/3 ñòðàíèö ïðåäëàãàþòñÿ íîâûå áîëåå
êîðîòêèå è ïðîñòûå äîêàçàòåëüñòâà.