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2/05/2554

Exercises in Graph Theory (9-2-1999, T.W.Leung)

Exercises in Graph Theory (9-2-1999, T.W.Leung)


1. In a party of n persons, , at least one person does not shake
hand with everybody, determine the maximum number of persons in the
party that shake hand with everybody.


2. Given a party of n persons. It is known that from every subgroup of
4 persons, there exists one who knows the other three persons.
Determine the minimum number of persons in the group who know
everybody.


3. In a conference there are 34 teams, each team consists of a leader
and a number. The participants shade hands with others, except the
leader of a team does not shake hand with his member. After many
hand-shakings the leader of team A asks everybody how many hands they
have shaken, and receives distinct answers, how many hand the member
of team A has shaken?


4. A contest consists of the first round and the final round, with
altogether 28 questions. It is known that each participant solves
exactly 7 questions, and every pair of questions is solved by exactly
two contestants. Show that there exists a contestant, who either does
not solve any question in the first round, or solves at least 4
questions in the first round.


5. There are n points ( ) in the plane and any pair of points is of
distance at least 1, show that there are at most pairs of points
whose distances are exactly 1.


6. There are 7 boys and 7 girls in a party, after that they record the
number of times they dance : 3, 3, 3, 3, 3, 5, 6, 6, 6, 6, 6, 6, 6, 6.
Show that someone has made a mistake.


7. There are n persons sit around a table ( ), show that one can
re-arrange the table so that the two persons besides every person are
different from the first siting.
8. There are n points on the circle, label them with 1, 2, , n, so
that any two adjacent points differ by at most 2. Can it be done? Is
it unique if it can?


9. Among 18 persons, there exists 4 persons who know each other, or
does not know each other. Show that.


10. Among 6 persons, there exist any least 2 groups of 3 persons, who
either know each other, or does not know each other.


11. There are n points on a plane, no three is collinear. Each point
is connected with at least k points, , show that there exist at least
one triangle.


12. It is given that points connected by at least edges, show that
there exists at least one triangle.


13. In a group of 1990 physicists, everyone has worked with at least
1327 physicists, show that there exists a group of 4 physicists, who
has worked with each other.


14. In a congregation of 500 participants, each participant knows
exactly 400 other participant, can we find 6 participants who know
each other? What happen if every participant know more than 400
participants?


15. If a graph G has n vertices, but contains no triangles nor
quadrilaterals, then G has at most edges.


16. A simple graph isomorphic to its complement is a
self-complementary graph. Suppose G is a self-complementary graph with
n vertices, show that or , give examples.

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