function f = caseI( w, x )
%
% evaluate the functions describing the eignenmode of case I,
% a uniform string.  argument w(3) is actually the eigenfrequency
% 

c_sq = 100.0;  %  the square of the wave speed, c^2
f1 = w(2);
f2 = - w(1)*w(3)^2/c_sq;
f3 = 0;       % never change the eigenvalue --- trick!

f = [ f1, f2, f3 ];
