Euler considers the following problem: for a given modulus N and integer a, find the smallest power an whose residue equals 1. Or, more generally, find the smallest power an that leaves a given residue r known today as the discrete logarithm problem.
Original Source Citation
Opuscula analytica, Volume 1, pp. 242-295.
Opera Omnia Citation
Series 1, Volume 4, pp.25-64.