def new_var(var_name, t_index, agent):
    """
    Naming of agent variables xij, where i in {1,...,T} and j in {1,...,M}
    :param var_name: variable name x, u, d,...
    :param t_index: time index
    :param agent: agent index
    :return: string with the name of the variable
    """
    return f"{var_name}{t_index}_{agent}"