function f = fun_cos(x,k)
%FUN_COS

if nargin < 2 | k == 0
   f = cos(x);
else
   g = mod(ceil(k/2),2);
   h = mod(k,2);
   if h == 1
      f = sin(x)*(-1)^g; 
   else
      f = cos(x)*(-1)^g; 
   end
end
