1. On some properties of the characteristic exponents of a system of ordinary linear differential equations with periodic coefficients;Demidovič, B. P.;Moskov. Gos. Univ. U\v{c}. Zap. Mat.,1952
2. Über Krümmung und Windung geschlossener Raumkurven;Fenchel, Werner;Math. Ann.,1929