Observationes generales circa series, quarum termini secundum sinus vel   cosinus angulorum multiplorum progrediuntur
  This paper, along with E592 and E636, seems to consider the binomial expansion (1+z)^n in the case where z is complex. Euler even gives the sums of divergent series. The paper is translated from Euler's Latin original into German. 
Leonhard Euler, Artur Diener, Alexander Aycock
math.HO