check_2F1_converges.hpp

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#ifndef STAN_MATH_PRIM_ERR_CHECK_2F1_CONVERGES_HPP
#define STAN_MATH_PRIM_ERR_CHECK_2F1_CONVERGES_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err/check_not_nan.hpp>
#include <stan/math/prim/fun/fabs.hpp>
#include <stan/math/prim/fun/floor.hpp>
#include <stan/math/prim/fun/is_nonpositive_integer.hpp>
#include <stan/math/prim/fun/value_of_rec.hpp>
#include <cmath>
#include <sstream>
#include <stdexcept>
 
namespace stan {
namespace math {
 
template <typename T_a1, typename T_a2, typename T_b1, typename T_z>
inline void check_2F1_converges(const char* function, const T_a1& a1,
                                const T_a2& a2, const T_b1& b1, const T_z& z) {
  using std::fabs;
  using std::floor;
 
  check_not_nan("check_3F2_converges", "a1", a1);
  check_not_nan("check_3F2_converges", "a2", a2);
  check_not_nan("check_3F2_converges", "b1", b1);
  check_not_nan("check_3F2_converges", "z", z);
 
  int num_terms = 0;
  bool is_polynomial = false;
 
  if (is_nonpositive_integer(a1) && fabs(a1) >= num_terms) {
    is_polynomial = true;
    num_terms = floor(fabs(value_of_rec(a1)));
  }
  if (is_nonpositive_integer(a2) && fabs(a2) >= num_terms) {
    is_polynomial = true;
    num_terms = floor(fabs(value_of_rec(a2)));
  }
 
  bool is_undefined = is_nonpositive_integer(b1) && fabs(b1) <= num_terms;
 
  if (!is_undefined
      && (is_polynomial || fabs(z) < 1 || (fabs(z) == 1 && b1 > a1 + a2))) {
    return;
  }
 
  std::stringstream msg;
  msg << "called from function '" << function << "', "
      << "hypergeometric function 2F1 does not meet convergence "
      << "conditions with given arguments. "
      << "a1: " << a1 << ", a2: " << a2 << ", "
      << "b1: " << b1 << ", z: " << z;
  throw std::domain_error(msg.str());
}
 
}  // namespace math
}  // namespace stan
#endif