# Proving correctness of connected algorithm

Okay, so a graph $G = (V, E)$ is (fully) connected if and only if for every pair of vertices $u, v ∈V,$ $u$ and $v$ are connected in $G$.

And that if the vertex set is $V = \{0, 1, . . . , n−1\}$, then we can represent a graph in a program as a two-dimensional n-by-n array $A$, where the entry $A[i][j]$ is set to $1$ if vertices $i$ and $j$ are adjacent (the edge), and $0$ if they aren’t. The following algorithm takes as input such an array, and returns True if the algorithm is connected, and False otherwise.

def connected(A):
n = len(A) # This is the number of vertices in the graph.

# First, set diagonal to 1, since every vertex is connected to itself.
for i in range(n):
A[i][i] = 1

for q in range(1, ceil(log(n)) + 1): # Main Loop: q =1,2,...,ceil(log(n))
A1 = new n by n array containing all zeros # UPDATE: Assume this takes n*n steps.
# Find new connectedness information
for i in range(n):
for j in range(n):
for k in range(n):
if A[i][k] == 1 and A[k][j] == 1:
A1[i][j] = 1 # UPDATE: A1 stores new connectivity
# information.

for i in range(n): # UPDATE: Change A to store the new connections.
for j in range(n):
if A[i][j] == 0: # If A[i][j] is already 1, don't need to update.
A[i][j] = A1[i][j]

# Check if the graph is connected at this point.
all_conn = True
for i in range(n):
if A[0][i] == 0:
all_conn = False
break

if all_conn:
return True
# The loop has ended and *not* returned early.
return False

So as you can see, this algorithm starts with an adjacency matrix $A$ and continually updates the entries of $A$ to represent new connectedness relationships between pairs of vertices. Essentially, at each iteration of the innermost loop, the algorithm checks whether vertex $k$ is connected to both $i$ and $j$; and if so, it records the fact that $i$ and $j$ are also connected.

I need to prove the the following predicate, where $G$ is a graph, $u$ and $v$ are vertices in the graph, and $d\in \mathbb{N}$.

$$PathLength(G,u,v,d):\text{"there is a path in G between u and v of length at most d"}$$

Using this predicate, we can state the key property of graphs that makes the connected algorithm work: $\forall G=(V,E), \forall u,v \in V, \forall d \in \mathbb{N}, PathLength(G,u,v,2d)\Leftrightarrow (\exists w \in V, PathLength(G, u,w,d) \wedge PathLength(G,w,v,d)$

Let $n\in \mathbb{Z^+}$ with $n>1$, and consider running connected on an n-by-n adjacency matrix representing a graph $G = (V,E)$, where $V = \{0,1,...,n-1\}$.

Prove by induction on $q$ that at the end of iteration $q$ of the Main Loop, the following is true:$$\forall i,j \in V, A[i][j] = 1 \Rightarrow PathLength(G, i, j, 2q)$$

• Have you done proofs by induction before? If not, learn the basics first (Jeff Erickson has some great online notes). If so, what will you want to use as the induction variable here? What base case(s) could be useful? – j_random_hacker Apr 8 '17 at 7:57
• Please clarify which parts are yours and which you copied, and from where. – Raphael Apr 8 '17 at 9:25