Let a and b be integers. We define a mod b to be the residue of dividing a by b. For example, if a evenly divides b, then a mod b=0, 20 mod 6= 2. The modulus operator is widely used in programming, and it is convenient when a and b are large numbers.
a mod b is always a nonnegative integer. In fact, 0≤ a mod b≤ |b-1| by the division algorithm. a and b can also be negative integers. Since 8=-(-5)+3 then 8 mod -5= 3.
As a final example, some known properties can be rewritten in terms of mod. a mod 2=0 if and only if a is even. a mod 2=1 if and only if a is odd.
