The spherical derivative of meromorphic functions in the neighbourhood of an isolated singularity. Olli Lehto — 1959 Commentarii mathematici Helvetici
On an extension of the concept of deficiency in the theory of meromorphic functions Olli LEHTO — 1953 Mathematica Scandinavica
Domain constants associated with Schwarzian derivative. Olli Lehto — 1977 Commentarii mathematici Helvetici