a = 0.9;
n = -10:20;
y = zeros(size(n));
for i = 1:length(n)
    if n(i)>=0, 
       y(i) = a^n(i); 
    end
end
axes('FontSize',18);
o = stem(n,y);
set(o(1),'Marker','.');
label('u1',18,'Time(n)','0.9^n1(n)',18)