Gauss claimed, but did not prove, that the condition was also necessary and several authors, notably Felix Klein, attributed this part of the proof to him as well.
Гаусс стверджував, але не доводив, що умова також необхідна, і кілька авторів, зокрема, Фелікс Кляйн, також приписують йому цю частину докази. attributed this part of the proof to him as well.