Solution to the problem of reciprocal trajectories
This article contains Euler's first published use of complex numbers and a many-axis geometric construction. Euler also defines even functions f(x) as those for which f(x) = f(-x), perhaps the first use of this term.
Original Source Citation
Commentarii academiae scientiarum Petropolitanae, Volume 2, pp. 90-111.
Opera Omnia Citation
Series 1, Volume 27, pp.6-23.