dfdxi

# File ext/bigdecimal/lib/bigdecimal/jacobian.rb, line 45
  def dfdxi(f,fx,x,i)
    nRetry = 0
    n = x.size
    xSave = x[i]
    ok = 0
    ratio = f.ten*f.ten*f.ten
    dx = x[i].abs/ratio
    dx = fx[i].abs/ratio if isEqual(dx,f.zero,f.zero,f.eps)
    dx = f.one/f.ten     if isEqual(dx,f.zero,f.zero,f.eps)
    until ok>0 do
      deriv = []
      nRetry += 1
      if nRetry > 100
        raise "Singular Jacobian matrix. No change at x[" + i.to_s + "]"
      end
      dx = dx*f.two
      x[i] += dx
      fxNew = f.values(x)
      for j in 0...n do
        if !isEqual(fxNew[j],fx[j],f.zero,f.eps) then
          ok += 1
          deriv <<= (fxNew[j]-fx[j])/dx
        else
          deriv <<= f.zero
        end
      end
      x[i] = xSave
    end
    deriv
  end