Data Structures, Algorithms, & Applications in C++
Chapter 16, Exercise 29

For array adjacency-lists, assume that the length field of each of the n + 1 arrays that make up a two-dimensional n x n array is of type int and that each int is 32 bits. The space needed for aList is 32(n + 1 + e) bits.

When the adjacency matrix is represented using one boolean for each entry, the space needed is n2 bits. In this case, the adjacency matrix uses less space provided e > n2/32 - n - 1.