double precision function f(x,y)

c General information:

c     Function f is an infinitely smooth, bounded, periodic but
c     not harmonic function of two arguments.

c Arguments:

c     Avoid typos:
      implicit none

c     The two arguments of f:
      double precision x,y

c Local variables:

c     Parameters of the function
      double precision c1,c2
      parameter (c1=2.3d0,c2=1.5d0)

c Executable statements:

c     Return the value of f:
      f=sin(sin(c1*x-y))*cos(c2*cos(x+y))

c Exit:

      return
      end