function x = x_t(t,k,a)
           %STAT Interesting statistics.
           x1 = rectpuls(k*t+a+0.5,1);
           x2 = k*t+a+1;
           x3 = rectpuls(k*t+a-0.5,1);
           x4 = -k*t-a+1;
           
           x = x1.*x2 + x3.*x4;
           
 end