hypergeometric_2F1.hpp

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#ifndef STAN_MATH_FWD_FUN_HYPERGEOMETRIC_2F1_HPP
#define STAN_MATH_FWD_FUN_HYPERGEOMETRIC_2F1_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/fwd/core.hpp>
#include <stan/math/fwd/fun/value_of.hpp>
#include <stan/math/prim/fun/grad_2F1.hpp>
#include <stan/math/prim/fun/hypergeometric_2F1.hpp>
 
namespace stan {
namespace math {
 
template <typename Ta1, typename Ta2, typename Tb, typename Tz,
          require_all_stan_scalar_t<Ta1, Ta2, Tb, Tz>* = nullptr,
          require_any_fvar_t<Ta1, Ta2, Tb, Tz>* = nullptr>
inline return_type_t<Ta1, Ta2, Tb, Tz> hypergeometric_2F1(const Ta1& a1,
                                                          const Ta2& a2,
                                                          const Tb& b,
                                                          const Tz& z) {
  using fvar_t = return_type_t<Ta1, Ta2, Tb, Tz>;
 
  auto a1_val = value_of(a1);
  auto a2_val = value_of(a2);
  auto b_val = value_of(b);
  auto z_val = value_of(z);
 
  auto grad_tuple = grad_2F1(a1, a2, b, z);
 
  typename fvar_t::Scalar grad = 0;
 
  if constexpr (is_autodiff_v<Ta1>) {
    grad += a1.d() * std::get<0>(grad_tuple);
  }
  if constexpr (is_autodiff_v<Ta2>) {
    grad += a2.d() * std::get<1>(grad_tuple);
  }
  if constexpr (is_autodiff_v<Tb>) {
    grad += b.d() * std::get<2>(grad_tuple);
  }
  if constexpr (is_autodiff_v<Tz>) {
    grad += z.d() * std::get<3>(grad_tuple);
  }
 
  return fvar_t(hypergeometric_2F1(a1_val, a2_val, b_val, z_val), grad);
}
 
}  // namespace math
}  // namespace stan
#endif