On generalized Darboux frame of a spacelike curve lying on a lightlike surface in Minkowski space E31 Dedicated to the memory of Professor Emeritus Krishan Lal Duggal

In this paper we introduce generalized Darboux frame of a spacelike curve alpha lying on a lightlike surface in Minkowski space E31 . We prove that alpha has two such frames and obtain generalized Darboux frame's equations. We find the relations between the curvature functions kg, kn , tau g of alpha with respect to its Darboux frame and the curvature functions k similar to g, k similar to n , tau similar to g with respect to generalized Darboux frames. We show that such frames exist along a spacelike straight line lying on a ruled surface which is not entirely lightlike, but contains some lightlike points. We define lightlike ruled surfaces on which the tangent and the binormal indicatrix of a null Cartan curve are the principal curvature lines having tau similar to g = 0 and give some examples.