ROOT

=ROOT(equations, variables)

=ROOT({"x**2 + y**2 - 1","x - y"}, {0.5,0.5})

=ROOT({"x**3 + y - 1","x + y**3 - 1"}, {0.7,0.7})

=ROOT({"x**2 + y**2 - 1","x - y"}, {-0.5,-0.5})

=ROOT({"x**3 + y - 1","x + y**3 - 1"}, {0.2,0.2})

import math
from typing import List, Union
 
import numpy as np
from scipy.optimize import root as scipy_root
 
 
Number = Union[int, float]
 
 
def root(equations: List[List[str]], variables: List[List[Number]]) -> Union[List[List[float]], str]:
    """Solve a square nonlinear system using SciPy's ``root`` solver.
 
    Args:
        equations: 2D list of equations written in terms of variable names such as ``x`` and
            ``y``. Each entry must be a string expression that evaluates to zero at the solution.
        variables: 2D list providing the initial guess for each variable. The number of variables
            must match the number of equations.
 
    Returns:
        Single-row 2D list containing the solution vector, or an error message string if
        validation fails or the solver does not converge.
 
    This example function is provided as-is without any representation of accuracy.
    """
    if not isinstance(equations, list) or len(equations) == 0:
        return "Invalid input: equations must be a 2D list of strings."
 
    # Flatten nested equation lists while retaining only string entries.
    flattened_equations: List[str] = []
    for row in equations:
        if not isinstance(row, list) or len(row) == 0:
            return "Invalid input: equations must be a 2D list of strings."
        for item in row:
            if isinstance(item, str) and item.strip() != "":
                flattened_equations.append(item)
    if len(flattened_equations) == 0:
        return "Invalid input: equations must contain at least one string expression."
 
    # Normalize variables which may be passed as a scalar by the Excel add-in
    def normalize_to_2d_list(value):
        if not isinstance(value, list):
            return [[value]]
        return value
 
    variables = normalize_to_2d_list(variables)
    if not isinstance(variables, list) or len(variables) == 0 or not isinstance(variables[0], list):
        return "Invalid input: variables must be a 2D list of numeric values."
 
    initial_guess = variables[0]
    if len(initial_guess) == 0:
        return "Invalid input: variables must contain at least one initial guess value."
 
    try:
        x0 = np.asarray([float(value) for value in initial_guess], dtype=float)
    except (TypeError, ValueError):
        return "Invalid input: variables must contain numeric values."
 
    equation_count = len(flattened_equations)
    if x0.size != equation_count:
        return (
            f"Invalid input: number of variables ({x0.size}) must match number of equations ({equation_count})."
        )
 
    # Generate readable variable names (x, y, z, x3, x4, ...)
    base_names = ["x", "y", "z"]
    variable_names: List[str] = []
    for index in range(equation_count):
        if index < len(base_names):
            variable_names.append(base_names[index])
        else:
            variable_names.append(f"x{index}")
 
    safe_globals = {
        name: getattr(math, name)
        for name in dir(math)
        if not name.startswith("_")
    }
    safe_globals.update({"np": np, "numpy": np})
 
    def _system(vector: np.ndarray) -> np.ndarray:
        local_context = {name: vector[i] for i, name in enumerate(variable_names)}
        residuals: List[float] = []
        for expression in flattened_equations:
            try:
                value = eval(expression, safe_globals, local_context)
            except Exception:
                return np.full(equation_count, np.nan, dtype=float)
            try:
                numeric_value = float(value)
            except (TypeError, ValueError):
                return np.full(equation_count, np.nan, dtype=float)
            if not math.isfinite(numeric_value):
                return np.full(equation_count, np.nan, dtype=float)
            residuals.append(numeric_value)
        return np.asarray(residuals, dtype=float)
 
    try:
        result = scipy_root(_system, x0=x0)
    except ValueError as exc:
        return f"root error: {exc}"
    except Exception as exc:
        return f"root error: {exc}"
 
    if not result.success or result.x is None:
        message = result.message if hasattr(result, "message") else "Solver did not converge."
        return f"root failed: {message}"
 
    if not np.all(np.isfinite(result.x)):
        return "root failed: solution contains non-finite values."
 
    solution = [float(value) for value in result.x]
    return [solution]