The greatest integer function [t] is discontinuous at every value of t that is an integer.
The function f(x)=[2x] is discontinuous when the argument 2x is an integer.
2x=n,n∈Z
x=2n,n∈Z
Using the notation aZ={ax:x∈Z}, the set of points of discontinuity is represented by taking a=21.
The set is 21Z.