double f6 ( double x )
{
  double value;

  value =     cos (       x ) 
        + 5 * cos ( 1.6 * x )
        - 2 * cos ( 2.0 * x )
        + 5 * cos ( 4.5 * x )
        + 7 * cos ( 9.0 * x );

  return value;
}