サブ　xxp.js　　　Last modified: Mar 25, 2004
// defined gxp

function xxp(x, is)
{
  var pi2, w, t, i
  pi2 = 0.398942280401432677940
  if (x == 0.0) {
    return 1
  }
  else if (is == 1) {
    return gxp(Math.sqrt(x))*2.0
  }
  else if (is == 2) {
    return Math.exp(-x/2.0)
  }
  else if ((is%2) == 0) {
    w = t = 1.0
    for (i = 2; i <= is-2; i += 2) {
      t *= x/i
      w += t
    }
    return Math.exp(Math.log(w) - x*0.5)
  }
  else {
    t = w = Math.sqrt(x)
    for (i = 3; i <= is-2; i += 2) {
      t *= x/i
      w += t
    }
    return 2.0*(gxp(Math.sqrt(x))+pi2*Math.exp(Math.log(w)-x*0.5))
  }
}