abstract class fi_xi extends Mathd
{
  // multi function  multi independent variable
  // an array value is returned 
  // example f=x[0]+sin(x[1])
  //         f[1]=x[0]*x[0]-x[1]
  // func(x,1) returns the value of f[1]
  // func(x,0) returns the value of f[0]
  abstract double[] func(double x[]);
}
