Write the set of integers that can be written as the difference of two integer squares" in a set builder notation, without using any words:
a) {x | x = n² - m², n, m ∈ ℤ}
b) {x | x = n² + m², n, m ∈ ℤ}
c) {x | x = 2n² - m², n, m ∈ ℤ}
d) {x | x = n³ - m², n, m ∈ ℤ}