def students_not_viewed(students, list_viewed):
    """
    Determine which students need to be send a message.

    :param students: List of the enrolled students.
    :type students: list(str)
    :param list_viewed: List of the students who have viewed a course
    :type list_viewed: list(str)
    :return: A list of student ids.
    :rtype: list(str)
    """
    return list(set(students) - set(list_viewed))