Why does fzero returns initial guess.

My script is below, whenever I enter this script into Matlab it returns to me my initial guess. I am curious Why? All help appreciated.

q = 10
theta = 135
Q = 600
z = 20

P = @(x,y) q*sin(theta)*x - q*cos(theta)*y - Q./(2*pi*z)*atan((x./y));
D = []
for x = -10:10;
    for y = -10:10;
        E= P(x,y);
        D = [D;E,x,y];
    end
end
%to check if the equation T (psi) works for root finding equation 
T = @(x,y) q*sin(theta)*x - q*cos(theta)*y - Q./(2*pi*z)*atan((x./y))-P(x,y);
L = [];
for x = -10:10;
    for y = -10:10;        
       Q=T(x,y);
       L = [L;Q];
    end
end

R=[]
x = [];
for y = -5:5;
[yi]= fzero(@(x) T(x,y),25);

x=[x;yi,y]
end

Answer 1

[Star Strider]

Your ‘T’ function is for all intents and purposes ‘P(x,y)-P(x,y)’. The fzero function detects zero-crossings, and ‘T’ is zero everywhere:

P = @(x,y) q*sin(theta)*x - q*cos(theta)*y - Q./(2*pi*z)*atan((x./y));
T = @(x,y) q*sin(theta)*x - q*cos(theta)*y - Q./(2*pi*z)*atan((x./y))-P(x,y);