1. Euclid did not have this notation to use. He described it in the following way: if the sum of a geometric sequence starting with 1 and having a ratio of 2 is prime, then multiplying this sum by the last element of the sequence yields a perfect number. (Euclid: Elements. Sir Thomas L. Heath, New York, Dover Publications, 1956.)
2. Cf. K. Härtig and J. Suranyi: Periodica Math. Hung. 6 (1975) pp. 235–240.