View source for Guichard congruence

A congruence of straight lines whose focal nets are formed by the curvature lines of the focal surfaces. One of the surfaces of centres of curvature of each focal surface carries a focal net consisting of geodesic lines. The spherical image of the developable surfaces of a Guichard congruence is a [[Chebyshev net|Chebyshev net]]. The focal surfaces of a Guichard congruence are called Guichard surfaces. Guichard congruences are named after G. Guichard (1889), who was the first to consider them.

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  S.P. Finikov,   "Theorie der Kongruenzen" , Akademie Verlag  (1959)  (Translated from Russian)</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  V.I. Shulikovskii,   "Classical differential geometry in a tensor setting" , Moscow  (1963)  (In Russian)</TD></TR></table>



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====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  G. Guichard,   "Surfaces rapporteés à leurs lignes asymptotiques et congruences rapporteés à leurs développables"  ''Ann. Sc. Ec. Norm. Sup. (3)'' , '''6'''  (1889)  pp. 333–348</TD></TR></table>