How to take the mean of matrix inside of another matrix?

Hi all,

I'm trying to code G(F(x)) by putting an analytical solution inside of a numerical one. So far, I have:

nnodes = 51;   
nelements = nnodes-1;
t = 0:.25:24   %time
F = zeros(length(t),nnodes)

%The first loop, over time(t) for analytical solution of F(t):

 for i=1:length(t)-1;
   F(i,;) = 2*t(i)    %a simplifed version of the actual equation
 end

%This gives a (97x51) F matrix - I know, it's quite large but necessary unless I change other variables in the code

%Second loop, where A,B and C are constants defined earlier in code

 for i=1:length(t)-1;
   for j = 1:nelements;
     G(j) = -(A * B .* mean(mean(F(j:j+1,j:j+1)))) + C;
     LocalElementMatrix = [1-G(j), -1+G(j), -1-G(j), 1+G(j)];
     %Local matrix goes inside of a global element matrix
   end
 end

%G(j) is a (97x1) matrix

%Am I going about this the correct way? My end goal is to combine an analytical solution into a numerical one. When I set G(j) to a constant value, my local matrices and the rest of my code performs correctly, so I think it has to be an issue with the way I'm setting up G - maybe with taking the mean(mean) of F.

Any help is greatly appreciated!