text: #ifndef STAN_MATH_FWD_MAT_FUN_MDIVIDE_RIGHT_TRI_LOW_HPP
  #define STAN_MATH_FWD_MAT_FUN_MDIVIDE_RIGHT_TRI_LOW_HPP
  
  #include <stan/math/prim/mat/fun/Eigen.hpp>
  #include <stan/math/prim/mat/fun/mdivide_right.hpp>
  #include <stan/math/prim/mat/err/check_multiplicable.hpp>
  #include <stan/math/prim/mat/err/check_square.hpp>
  #include <stan/math/fwd/mat/fun/to_fvar.hpp>
  #include <stan/math/fwd/mat/fun/multiply.hpp>
  #include <stan/math/fwd/core.hpp>
  
  namespace stan {
  namespace math {
  
  template <typename T, int R1, int C1, int R2, int C2>
  inline Eigen::Matrix<fvar<T>, R1, C1> mdivide_right_tri_low(
      const Eigen::Matrix<fvar<T>, R1, C1> &A,
      const Eigen::Matrix<fvar<T>, R2, C2> &b) {
    check_square("mdivide_right_tri_low", "b", b);
    check_multiplicable("mdivide_right_tri_low", "A", A, "b", b);
  
    Eigen::Matrix<T, R1, C2> A_mult_inv_b(A.rows(), b.cols());
    Eigen::Matrix<T, R1, C2> deriv_A_mult_inv_b(A.rows(), b.cols());
    Eigen::Matrix<T, R2, C2> deriv_b_mult_inv_b(b.rows(), b.cols());
    Eigen::Matrix<T, R1, C1> val_A(A.rows(), A.cols());
    Eigen::Matrix<T, R1, C1> deriv_A(A.rows(), A.cols());
    Eigen::Matrix<T, R2, C2> val_b(b.rows(), b.cols());
    Eigen::Matrix<T, R2, C2> deriv_b(b.rows(), b.cols());
    val_b.setZero();
    deriv_b.setZero();
  
    for (size_type j = 0; j < A.cols(); j++) {
      for (size_type i = 0; i < A.rows(); i++) {
        val_A(i, j) = A(i, j).val_;
        deriv_A(i, j) = A(i, j).d_;
      }
    }
  
    for (size_type j = 0; j < b.cols(); j++) {
      for (size_type i = j; i < b.rows(); i++) {
        val_b(i, j) = b(i, j).val_;
        deriv_b(i, j) = b(i, j).d_;
      }
    }
  
    A_mult_inv_b = mdivide_right(val_A, val_b);
    deriv_A_mult_inv_b = mdivide_right(deriv_A, val_b);
    deriv_b_mult_inv_b = mdivide_right(deriv_b, val_b);
  
    Eigen::Matrix<T, R1, C2> deriv(A.rows(), b.cols());
    deriv = deriv_A_mult_inv_b - multiply(A_mult_inv_b, deriv_b_mult_inv_b);
  
    return to_fvar(A_mult_inv_b, deriv);
  }
  
  template <typename T, int R1, int C1, int R2, int C2>
  inline Eigen::Matrix<fvar<T>, R1, C2> mdivide_right_tri_low(
      const Eigen::Matrix<fvar<T>, R1, C1> &A,
      const Eigen::Matrix<double, R2, C2> &b) {
    check_square("mdivide_right_tri_low", "b", b);
    check_multiplicable("mdivide_right_tri_low", "A", A, "b", b);
  
    Eigen::Matrix<T, R2, C2> deriv_b_mult_inv_b(b.rows(), b.cols());
    Eigen::Matrix<T, R1, C1> val_A(A.rows(), A.cols());
    Eigen::Matrix<T, R1, C1> deriv_A(A.rows(), A.cols());
    Eigen::Matrix<T, R2, C2> val_b(b.rows(), b.cols());
    val_b.setZero();
  
    for (int j = 0; j < A.cols(); j++) {
      for (int i = 0; i < A.rows(); i++) {
        val_A(i, j) = A(i, j).val_;
        deriv_A(i, j) = A(i, j).d_;
      }
    }
  
    for (size_type j = 0; j < b.cols(); j++) {
      for (size_type i = j; i < b.rows(); i++) {
        val_b(i, j) = b(i, j);
      }
    }
  
    return to_fvar(mdivide_right(val_A, val_b), mdivide_right(deriv_A, val_b));
  }
  
  template <typename T, int R1, int C1, int R2, int C2>
  inline Eigen::Matrix<fvar<T>, R1, C2> mdivide_right_tri_low(
      const Eigen::Matrix<double, R1, C1> &A,
      const Eigen::Matrix<fvar<T>, R2, C2> &b) {
    check_square("mdivide_right_tri_low", "b", b);
    check_multiplicable("mdivide_right_tri_low", "A", A, "b", b);
  
    Eigen::Matrix<T, R1, C2> A_mult_inv_b(A.rows(), b.cols());
    Eigen::Matrix<T, R2, C2> deriv_b_mult_inv_b(b.rows(), b.cols());
    Eigen::Matrix<T, R2, C2> val_b(b.rows(), b.cols());
    Eigen::Matrix<T, R2, C2> deriv_b(b.rows(), b.cols());
    val_b.setZero();
    deriv_b.setZero();
  
    for (int j = 0; j < b.cols(); j++) {
      for (int i = j; i < b.rows(); i++) {
        val_b(i, j) = b(i, j).val_;
        deriv_b(i, j) = b(i, j).d_;
      }
    }
  
    A_mult_inv_b = mdivide_right(A, val_b);
    deriv_b_mult_inv_b = mdivide_right(deriv_b, val_b);
  
    Eigen::Matrix<T, R1, C2> deriv(A.rows(), b.cols());
    deriv = -multiply(A_mult_inv_b, deriv_b_mult_inv_b);
  
    return to_fvar(A_mult_inv_b, deriv);
  }
  
  }  // namespace math
  }  // namespace stan
  #endif
  