inv_sqrt.hpp

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#ifndef STAN_MATH_REV_FUN_INV_SQRT_HPP
#define STAN_MATH_REV_FUN_INV_SQRT_HPP
 
#include <stan/math/rev/meta.hpp>
#include <stan/math/rev/core.hpp>
#include <stan/math/prim/fun/inv_sqrt.hpp>
#include <stan/math/rev/fun/to_arena.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
template <typename T, require_stan_scalar_or_eigen_t<T>* = nullptr>
inline auto inv_sqrt(const var_value<T>& a) {
  auto denom = to_arena(as_array_or_scalar(a.val())
                        * as_array_or_scalar(sqrt(a.val())));
  return make_callback_var(inv_sqrt(a.val()), [a, denom](auto& vi) mutable {
    as_array_or_scalar(a.adj()) -= 0.5 * as_array_or_scalar(vi.adj()) / denom;
  });
}
 
}  // namespace math
}  // namespace stan
#endif