
def evaluate_expression(expression):
    """
    Evaluates a mathematical expression with support for floor function notation and power notation.

    Args:
        expression (str): The mathematical expression to evaluate. It can only contain the symbol 'x'.

    Returns:
        Union[sympy.Expr, str]: The evaluated result as a sympy expression if successful,
        otherwise an error message as a string.

    """
    from sympy import sympify, symbols

    # Replace power with ** for sympy
    expression = expression.replace('^', '**')
    # Replace the floor function notation
    expression = expression.replace('\\lfloor', 'floor(').replace('\\rfloor', ')')
    try:
        # Create a symbol 'x' for use in case it is in the expression
        x = symbols('x')
        # Evaluate the expression
        result = sympify(expression)
        return result
    except Exception as e:
        return str(e)