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    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
    (Scientific and Technological Research Council Turkey, 2023) Djordjevic, Jelena; Nesovic, Emilija; Öztürk, Ufuk
    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.
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    On null Cartan normal isophotic and normal silhouette curves on a timelike surface in Minkowski 3-space
    (Wiley, 2024) Djordjevic, Jelena; Nesovic, Emilija; Ozturk, Ufuk; Ozturk, Esra B. Koc
    We introduce generalized Darboux frames along a null Cartan curve lying on a timelike surface in Minkowski space E-1(3) and define null Cartan normal isophotic and normal silhouette curves in terms of the vector field that lies in the normal plane of the curve and belongs to its generalized Darboux frame of the first kind. We investigate null Cartan normal isophotic and normal silhouette curves with constant geodesic curvature kg and constant geodesic torsion tau(g). We obtain the parameter equations of their axes and prove that such curves are the null Cartan helices or the null Cartan cubics. In particular, we show that null Cartan normal isophotic curves with a non-zero constant curvatures k(g) and tau(g) have a remarkable property that they are general helices, relatively normal-slant helices and isophotic curves with respect to the same axis. We prove that null Cartan cubics lying on a timelike surface are normal isophotic curves with a spacelike axis and normal silhouette curves with a lightlike axis. We obtain the relation between Minkowski Pythagorean hodograph cubic curves and null Cartan normal isophotic and normal silhouette curves. Finally, we give numerical examples of null Cartan normal isophotic and normal silhouette curves obtained by integrating the system of two the first order differential equations under the initial conditions.