Writing short function that uses function handle in output?

I'm attempting to write a function in MATLAB that will output out the double series formula (seen in comment below as well as the image below, ignoring the z and phi terms)

In short, what I would like for this function to do is to take in an integer number of k and n as arguments in and generate a formula containing unknown parameters A(k,n), theta(k), and theta(n) that I can eventually solve for using lsqcurvefit(). Right now, all the current function outputs is:

 @(A,t)A(ik,in).*(cos((2*pi*ik*Vs*t)/lambda+thetak(ik)).*cos((2*pi*in*t)/tau+thetan(in)))

Is this because of the function handle? Is there a more efficient way for me to translate my formula into code?

fun = genfunction(k,n)
% WIP as of 6/18: long stretch goal is to have this also output an array of
% x0 guesses:
% [formula, x0] = genfunction(k,n)

 %Takes number of k and n to generate formula in the form:
 % sum(k=1 to k)sum(n=1 to n)A(k,n)cos(2pi*k*Vs*t/lambda+theta(k))*cos(2pi*n*t/tau+theta(n)) 

Vs = 7.6; %m/s
lambda = 20; %m
tau = 20; %s

 A = zeros(k,n);
 thetak = zeros(1,k);
 thetan = zeros(1,n);

 %fun = @(A,t)A*(cos((2*pi*Vs*t)/lambda + thetak)*cos((2*pi*t)/tau + thetan));
 fun = 0;
 for ik=1:k
    for in=1:n
       fun = @(A,t) A(ik,in).*(cos((2*pi*ik*Vs*t)/lambda + thetak(ik)).*cos((2*pi*in*t)/tau + thetan(in)));
    end 
end