# CS 4104 Homework Assignment 2

CS 4104
Homework Assignment 2
Given: September 3, 2019 Due: September 13, 2019
General directions. The point value of each problem is shown in [ ]. Each solution must
include all details and an explanation of why the given solution is correct. In particular,
write complete sentences. A correct answer without an explanation is worth
no credit. The completed assignment must be submitted on Canvas as a PDF by 5:00
PM on September 13, 2019. No late homework will be accepted.
Digital preparation of your solutions is mandatory. Use of LATEX is optional, but
Use of LATEX (optional, but encouraged).
• Retrieve this LATEX source file, named homework2.tex, from the course web site.
• Rename the file _solvehw2.tex, For example, for the instructor,
the file name would be heath_solvehw2.tex.
• Use a text editor (such as vi, emacs, or pico) to accomplish the next three steps.
• Uncomment the line
% setboolean{solutions}{True}
in the document preamble by deleting the %.
• Find the line
renewcommand{author}{Lenwood S. Heath}
and replace the instructor’s name with your name.
• Generate a PDF and turn it in on Canvas by 5:00 PM on September 13, 2019.
2 Homework Assignment 2 September 3, 2019
Comment. This assignment should be more challenging than Homework Assignment 1,
so please start working on it early. It continues the use of writing in complete sentences
and writing in formal mathematical notation and begins the use of drawing digitally, in this
case to draw graphs without and with weights on vertices. As usual, pay careful attention
to the directions and ask questions on Piazza or at office hours.
It will be helpful to draw small examples of graphs to gain intuition of what is being
defined.
If you use LATEX, you can find many useful LATEX examples in the .tar files for the
lecture notes for things like pseudocode and formally stating a problem.
 1. For each integer n satisfying n ≥ 1, let Vn = {v1, v2, . . . , vn} be a set of n vertices.
Let k be an integer satisfying 0 ≤ k ≤ n − 1. The n, k-braid is the undirected graph
Gn,k = (Vn,En,k) where
En,k = {(vi, vj) | i 6= j and |j − i| ≤ k}.
A free set in Gn,k is a set U ⊆ Vn such that no two vertices in U are adjacent. U is a
maximal free set if it is not possible to add a vertex to U and continue to have a free set.
A weight function on Gn,k is a function w : Vn → N, where N = {0, 1, 2, . . .}. If U ⊆ Vn,
then w(U) = Pvi2U w(vi).
A. Draw G7,2, and insert your drawing into your solutions. For this drawing,
you do not need vertex weights. List three maximal free sets for G7,2.
B. The Free Set problem takes as an instance a braid Gn,k and a weight
function w on Gn,k and returns as a solution a free set U such that w(U) is
maximum among all free sets in Gn,k. State the Free Set problem in the
formal instance/solution format that we use in class.
C. Figure 1 contains pseudocode for a greedy algorithm Greedy-Free-Set
that attempts to solve the Free Set problem. Draw an instance Gn,k,w of
Free Set for which Greedy-Free-Set does not return a correct solution.
(Here, you need to assign weights to vertices in your drawing.) Explain
D. Use the dynamic programming paradigm from class to develop a dynamic
programming algorithm to solve the Free Set problem for the special case
where k = 1. Give CLRS pseudocode for your algorithm. You may skip
the backtrace step. Give the £ asymptotic worst-case time complexity for
E. Use the dynamic programming paradigm from class to develop a dynamic
programming algorithm to solve the Free Set problem for the special case
where k = 2. Give CLRS pseudocode for your algorithm. You may skip
the backtrace step. Give the £ asymptotic worst-case time complexity for
September 3, 2019 Homework Assignment 2 3
Greedy-Free-Set(Gn,k,w)
1 // Gn,k is the n, k-braid, and w is a weight function on Gn,k.
2 // We attempt to return a free set U in Gn,k of maximum weight.
3 U = ∅
4 X = Vn
5 while X 6= ∅
6 select vi ∈ X of maximum weight w(vi)
7 U = U ∪ {vi}
8 delete vi and all neighbors of vi from X
9 return U
Figure 1: Pseudocode for a greedy algorithm for the Free Set problem.

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