% This is the function invoked by the code  hw1_EX_Euler_callsfun.m, which 
%  in its turn involves  hw1_EX_main4Euler.m

function f = fun4_hw1_EX_main4Euler(x,y);

global a b             % The meaning of this line is explained in the main code.
                       % The global variables must be declared as such *both* 
                       % in the main code (hw1_EX_main4Euler.m  in this case)
                       % and in the actual function that uses them. 

f = a*x + b*y;