N=50;
my=10/7+4/7*(-0.4).^[0:N-1];
n=[0:N-1];
stem(n,my); pause
b=2;
a=conv([1 0.4], [1 -1]);
y=impz(b,a,N);
stem(n,y,'m'); pause
stem(n,y-my.'); pause