On the multiplication of angles which are to be obtained by factors
This paper, along with E592 and E655, seems to consider the binomial expansion of (1+z)n in the case where z is complex.
Original Source Citation
Nova Acta Academiae Scientiarum Imperialis Petropolitanae, Volume 5, pp. 27-51.
Opera Omnia Citation
Series 1, Volume 16A, pp.79-111.