code: function _putRho(s, k, t, v, r)
  {
    var w = bs.getW(s, k, t, v, r);
    if(!isNaN(w))
    {
      return -1 * k * t * Math.pow(Math.E, -1 * r * t) * bs.stdNormCDF(v * Math.sqrt(t) - w);
    }
    else
    {
      return 0;
    }
  }
docs: Calculates the rho of a put option.
  
  @private
  @param {Number} s Current price of the underlying
  @param {Number} k Strike price
  @param {Number} t Time to experiation in years
  @param {Number} v Volatility as a decimal
  @param {Number} r Anual risk-free interest rate as a decimal
  @returns {Number} The rho of the put option