ction [u, X_, Y_] = solver_4_2_5( dx, dy, dt, slices )
%       f_ = @(X, Y)( zeros(size(X)) );
%       F_ = @(X, Y, t)( zeros(size(X)) );
%       g_0t = @(W, t)( sin(t) .* sin(pi * W) );
%       g_1t = @(W, t)( zeros(size(W)) );
%       nu = 1;
%       slices_id = round(slices / dt);
%   
%       X = round(1 / dx);
%       Y = round(1 / dy);
%       x = 1; y = 1;
%       Mx = x * X; My = y * Y;
%       
%       rx = nu * dt / dx / dx;
%       ry = nu * dt / dy / dy;
%   
%       x_ = 0 : dx : x;
%       y_ 