One‐parameter families of Legendre curves and plane line congruences

Families of curves in the Euclidean plane naturally contain singular curves, where the frame of classical differential geometry does not work well. We introduce the notions of one‐parameter family of Legendre curves in the Euclidean plane, congruent equivalence and curvature. Especially, a one‐parameter family of Legendre curves can contain singular curves, and is determined by the curvature up to congruence. We also give properties of one‐parameter families of Legendre curves. As applications, we give a relation between one‐parameter families of Legendre curves and Legendre surfaces. Moreover, we study plane line congruences (one‐parameter families of lines in plane) in terms of the curvatures as one‐parameter families of Legendre curves.