Fourier representation in for loop

i want to represent the square wave of 8 bit ASCII char 'b' in Fourier, i already derived the coefficients but i don't know why the square wave is not being represented ! plz support asap.

X = [0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8];

Y = [0 0 1 1 1 1 0 0 0 0 0 0 1 1 0 0];

L = 8/2;

n = 1:2:50;

figure(1);

plot (X,Y);

hold on;

grid on;

%x =linspace(0,2,10000);

%x= 1:0.1:2*pi;

%----Fourier-----

a_zero = 3/8;

f = zeros(1,length(Y));

x = ((2 .* (0:length(Y)-1)) + 1)./2;

x = x .* Y;

x = (x .* pi)./L;

%---For Loop ----

for i = 1:length(Y),

      for j= 1:length(n) 

          a_n = (1/pi*j)*(sin((3*pi*j)/4)- sin((pi*j)/4)+sin((7*pi*j)/4)-sin((6*pi*j)/4));

          b_n = (1/pi*j)*(cos((pi*j)/4)- cos((3*pi*j)/4)+cos((6*pi*j)/4)-cos((7*pi*j)/4));

          f(i) = f(i) + (a_n *cos((j*pi*X(i))/4)) + (b_n*sin((j*pi*X(i))/4));

      end

end

fx = f + a_zero;

plot(fx)