# Catalan surface

A ruled surface whose rectilinear generators are all parallel to the same plane. Its line of restriction (cf. Ruled surface) is planar. The position vector of a Catalan surface is $r=\rho(u)+vl(u)$, where $l''(u)\neq0$, $(l,l',l'')=0$. If all the generators of a Catalan surface intersect the same straight line, then the surface is a conoid.

#### References

 [1] E. Catalan, "Mémoire sur les surfaces gauches à plan directeur" , Paris (1843)