text: #ifndef STAN_MATH_REV_SCAL_FUN_POW_HPP
  #define STAN_MATH_REV_SCAL_FUN_POW_HPP
  
  #include <stan/math/rev/meta.hpp>
  #include <stan/math/rev/core.hpp>
  #include <stan/math/rev/scal/fun/inv.hpp>
  #include <stan/math/rev/scal/fun/inv_sqrt.hpp>
  #include <stan/math/rev/scal/fun/inv_square.hpp>
  #include <stan/math/rev/scal/fun/sqrt.hpp>
  #include <stan/math/rev/scal/fun/square.hpp>
  #include <stan/math/prim/scal/fun/is_any_nan.hpp>
  #include <cmath>
  #include <limits>
  
  namespace stan {
  namespace math {
  
  namespace internal {
  class pow_vv_vari : public op_vv_vari {
   public:
    pow_vv_vari(vari* avi, vari* bvi)
        : op_vv_vari(std::pow(avi->val_, bvi->val_), avi, bvi) {}
    void chain() {
      if (unlikely(is_any_nan(avi_->val_, bvi_->val_))) {
        avi_->adj_ = std::numeric_limits<double>::quiet_NaN();
        bvi_->adj_ = std::numeric_limits<double>::quiet_NaN();
      } else {
        if (avi_->val_ == 0.0) {
          return;  // partials zero, avoids 0 & log(0)
        }
        avi_->adj_ += adj_ * bvi_->val_ * val_ / avi_->val_;
        bvi_->adj_ += adj_ * std::log(avi_->val_) * val_;
      }
    }
  };
  
  class pow_vd_vari : public op_vd_vari {
   public:
    pow_vd_vari(vari* avi, double b)
        : op_vd_vari(std::pow(avi->val_, b), avi, b) {}
    void chain() {
      if (unlikely(is_any_nan(avi_->val_, bd_))) {
        avi_->adj_ = std::numeric_limits<double>::quiet_NaN();
      } else {
        if (avi_->val_ == 0.0) {
          return;  // partials zero, avoids 0 & log(0)
        }
        avi_->adj_ += adj_ * bd_ * val_ / avi_->val_;
      }
    }
  };
  
  class pow_dv_vari : public op_dv_vari {
   public:
    pow_dv_vari(double a, vari* bvi)
        : op_dv_vari(std::pow(a, bvi->val_), a, bvi) {}
    void chain() {
      if (unlikely(is_any_nan(bvi_->val_, ad_))) {
        bvi_->adj_ = std::numeric_limits<double>::quiet_NaN();
      } else {
        if (ad_ == 0.0) {
          return;  // partials zero, avoids 0 & log(0)
        }
        bvi_->adj_ += adj_ * std::log(ad_) * val_;
      }
    }
  };
  }  // namespace internal
  
  /**
   * Return the base raised to the power of the exponent (cmath).
   *
   * The partial derivatives are
   *
   * \f$\frac{\partial}{\partial x} \mbox{pow}(x, y) = y x^{y-1}\f$, and
   *
   * \f$\frac{\partial}{\partial y} \mbox{pow}(x, y) = x^y \ \log x\f$.
   *
   *
     \f[
     \mbox{pow}(x, y) =
     \begin{cases}
       x^y & \mbox{if } -\infty\leq x, y \leq \infty \\[6pt]
       \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
     \end{cases}
     \f]
  
     \f[
     \frac{\partial\, \mbox{pow}(x, y)}{\partial x} =
     \begin{cases}
       yx^{y-1} & \mbox{if } -\infty\leq x\leq \infty \\[6pt]
       \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
     \end{cases}
     \f]
  
     \f[
     \frac{\partial\, \mbox{pow}(x, y)}{\partial y} =
     \begin{cases}
       x^y\ln x & \mbox{if } -\infty\leq x\leq \infty \\[6pt]
       \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
     \end{cases}
     \f]
   *
   * @param base Base variable.
   * @param exponent Exponent variable.
   * @return Base raised to the exponent.
   */
  inline var pow(const var& base, const var& exponent) {
    return var(new internal::pow_vv_vari(base.vi_, exponent.vi_));
  }
  
  /**
   * Return the base variable raised to the power of the exponent
   * scalar (cmath).
   *
   * The derivative for the variable is
   *
   * \f$\frac{d}{dx} \mbox{pow}(x, c) = c x^{c-1}\f$.
   *
   * @param base Base variable.
   * @param exponent Exponent scalar.
   * @return Base raised to the exponent.
   */
  inline var pow(const var& base, double exponent) {
    if (exponent == 0.5) {
      return sqrt(base);
    }
    if (exponent == 1.0) {
      return base;
    }
    if (exponent == 2.0) {
      return square(base);
    }
    if (exponent == -2.0) {
      return inv_square(base);
    }
    if (exponent == -1.0) {
      return inv(base);
    }
    if (exponent == -0.5) {
      return inv_sqrt(base);
    }
    return var(new internal::pow_vd_vari(base.vi_, exponent));
  }
  
  /**
   * Return the base scalar raised to the power of the exponent
   * variable (cmath).
   *
   * The derivative for the variable is
   *
   * \f$\frac{d}{d y} \mbox{pow}(c, y) = c^y \log c \f$.
   *
   * @param base Base scalar.
   * @param exponent Exponent variable.
   * @return Base raised to the exponent.
   */
  inline var pow(double base, const var& exponent) {
    return var(new internal::pow_dv_vari(base, exponent.vi_));
  }
  
  }  // namespace math
  }  // namespace stan
  #endif
  