how to keep a variable constant and other variable will change between two variable in the same code?

( I'm new here. If I make any mistake, please consider it.)

I got my solution with two variable. I want to draw 3D curve. where it holds two conditions.

1) Where a and b both are constant. I did this code:

x=0:.01:2;
y=0:.01:2;
b=0.5;[X, Y]=meshgrid(x, y);
uxy=zeros(size(X));
a=1;
m=a^2; 
pn=(16*m*sinh(pi*Y/2*a).*cos(pi*X/2*a))/(pi^3*sinh(pi*b/2));
uxy=(m- X.^2)/2-pn;
surf(X, Y,uxy)

Is it right or wrong? If not, then how can I do it?

2) I want to keep b as a constant. but var=a will change. I did this code:

x=0:.01:2;
y=0:.01:2;
b=0.5;
[X, Y] = meshgrid(x, y);
uxy=zeros(size(X));
for a=1:2
    m=a^2; 
    pn=(16*m*sinh(pi*Y/2*a).*cos(pi*X/2*a))/(pi^3*sinh(pi*b/2));
    uxy=(m- X.^2)/2-pn;
end
surf(X, Y, uxy)

Is it right or wrong? if not, then how can I do it?