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MATH2069/MATH2969 Discrete Mathematics and Graph 2018
发布时间:2022-06-17
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EXAMINATION
Semester 1 - Main, 2018
MATH2069/MATH2969 Discrete Mathematics and Graph
1. In this question, numerical answers need not be evaluated.
Suppose that there are 77 students enrolled in a course.
(a) What is the number of ways to select seven representatives to a student forum?
(b) Each of the students must attend a practice session in one of four dierent locations. What is the number of ways to assign the students to the practice sessions so that each session is attended by at least one student?
(c) The students were born in 10 dierent countries. Explain brie y why at least 8 of them were born in the same country.
(d) The class received a pack of 100 identical pens from a sponsor. What is the number of ways to distribute the pens among the students so that no one misses out?
2. (a) Use characteristic equation to nd the general solution of the recurrence relation
bn = 2 bn−2, where n > 2.
(b) Find the solution of the recurrence relation
an = 2 an−2 +n ( 1)n , where n > 2, satisfying the initial conditions a0 = 2 and a1 = 5.
(c) Use your solution to part (b) or otherwise to write down a closed formula for the generating function of the sequence an de ned by the recurrence relation in part (b).
3. This question is for MATH2069 students only.
(a) Prove that for any given formal power series A(z) there exists a unique formal power series B(z) with zero constant term such that B′ (z) = A(z).
(b) The generating function of a sequence an is given by the formula
A(z) = 1
Write down an explicit formula for an .
(c) Use your solution to part (b) or otherwise to nd a closed formula for the unique formal power series B(z) provided by the result in part (a).
(d) Find a closed formula for the unique formal power series C(z) with zero constant term, provided by the result in part (a), such that C′ (z) = B(z).
3. This question is for MATH2969 students only.
(a) Let F (z) be a given formal series and let G(z) be the unique formal power series with zero constant term such that G′ (z) = F (z). Prove that for a given value of a0 , the dierential equation A′ (z) = F (z)A(z) has a unique solution which is given by A(z) = a0 exp G(z) .
(b) Show that the formal power series A(z) = exp log(1 + z + z2 ) satis es the
equation A′ (z) = A(z) 1 + 2z
(c) Using the results in parts (a) and (b), prove the identity exp log(1 + z + z2 ) = 1 + z + z2 .
4. (a) Explain brie y why neither of the sequences (1, 1, 1, 1, 5, 5) and (1, 1, 1, 2, 3, 4, 5) is graphic.
(b) Show that the sequence (1, 1, 2, 2, 3, 3) is graphic by drawing a graph with this degree sequence.
(c) Show that it is possible to nd at least two non-isomorphic graphs with the degree sequence (1, 1, 2, 2, 3, 3). Justify your answer.
(d) What is the total number of isomorphism classes of graphs with the degree sequence (1, 1, 2, 2, 3, 3)? Justify your answer.
5. (a) Complete the formulation of the Travelling Salesman Problem: “Given a connected weighted graph, nd a walk which . . . ” .
(b) Find a walk which solves the Travelling Salesman Problem for the following weighted graph. Justify your answer.
b 17 c
f
(c) Find a minimal spanning tree for this weighted graph.
(d) Explain brie y why any connected graph with n vertices and n edges has a cycle.
(e) Use Cayley’s formula to derive that the number of connected graphs with the vertex set V = {1, 2, . . . , n}, which have n edges, does not exceed 1
6. This question is for MATH2069 students only. (a) Consider the following graph G:
(i) Find the chromatic number of G and produce a vertex colouring with the minimal number of colours.
(ii) What is the edge chromatic number of G? Justify your answer.
(b) Find all forests G (up to isomorphism) with the chromatic polynomial of the form PG (t) = t5 + a t4 + b t3 .
(c) Can you nd a graph with the chromatic polynomial of the form PG (t) = t5 + a t4 + b t3 which is not a forest?
6. This question is for MATH2969 students only.
Suppose that the chromatic number of a graph G with n vertices is n 1. (a) Show that the number of connected components of G is at most 2. (b) Show that if G is connected, then it contains a vertex of degree n 1.
(c) Find all possible chromatic polynomials PG (t) of the graph G.