inv_sqrt.hpp

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#ifndef STAN__AGRAD__FWD__FUNCTIONS__INV_SQRT_HPP
#define STAN__AGRAD__FWD__FUNCTIONS__INV_SQRT_HPP

#include <stan/agrad/fwd/fvar.hpp>
#include <stan/meta/traits.hpp>
#include <boost/math/tools/promotion.hpp>

namespace stan {

  namespace agrad {

    template <typename T>
    inline
    fvar<T>
    inv_sqrt(const fvar<T>& x) {
      using std::sqrt;
      T sqrt_x(sqrt(x.val_));
      return fvar<T>(1 / sqrt_x, -0.5 * x.d_ / (x.val_ * sqrt_x));
    }
  }
}
#endif