用matlab画图，要求如下图中展示的

t = -3:0.001:3;
N = [3 9 27 81];
T = 2;

for ii = 1:length(N)
    n = N(ii);
    k = (1:n)';
    A = repmat( ( (-1).^(k+1) ./ k ),1,length(t) ) .* sin( k.*pi*t/T );
    F(n,:) = (2*T/pi ) * sum(A);
end

figure
plot( t',F' )
legend('n=3','n=9','n=27','n=81')