サブ　pxf.js　　　Last modified: Mar 25, 2004
// defined fxp
// defined pxt
// defined pxx

function pxf(p0, df1, df2)
{
  var xl, xr, p9, temp, c, xm
  var ok = 0

  /* 二分法のための左右点を求める */
  xl = 0.0
  xr = 10.0
  while (xr < 1e30) {
    p9 = fxp(xr, df1, df2)
    if (p9 == p0) {
      return xr
    }
    else if (p9 < p0) {
      ok = 1
      break
    }
    xl = xr
    xr *= 10.0
  }

  if (ok == 0) {
    if (df1 > df2) {
      temp = df1
      df1 = df2
      df2 = temp
      p0 = 1.0-p0
      ok = 1
    }
    if (df1 == 1) {
      c = pxt(p0, df2)
      c = c*c
    }
    else {
      temp = df1-2.0
      c = pxx(p0, df1)
      c = c/df1*(1.0+ (c-temp)*0.5/df2 + (4.0*c*c-11.0*temp*c+temp*(7.0*df1-10.0))/(24.0*df2*df2)
        +(2.0*c*c*c-10.0*temp*c*c+temp*(17.0*df1-26.0)*c-(df1-2)*temp*(9.0*df1-6.0))/(48.0*df2*df2*df2))
    }
    return (ok == 1 && c != 0.0) ? 1.0/c : c
  }

  /* 二分法によりパーセント点を求める */
  for (;;) {
    xm = (xl+xr)*0.5
    p9 = fxp(xm, df1, df2)
    if (p9 > p0) {
      xl = xm
    }
    else {
      xr = xm
    }
    if (Math.abs(xr-xl)/xm < 1e-10) break
  }

  return xm
}