**Question:** I'd like to know if there is some reference or reasonable way to develop curve theory in a plane with degenerate metric $(\Bbb R^2, {\rm d}s^2 ={\rm d}x^2)$.

**Context:** In Lorentz-Minkowski space $\Bbb L^3 = (\Bbb R^3, \langle \cdot,\cdot\rangle_L = {\rm d}x^2+{\rm d}y^2 - {\rm d}z^2)$ one can consider unit-speed spacelike curves $\alpha\colon I \to \Bbb L^3$ with lightlike normal direction (meaning $T(s) = \alpha'(s)$, $N(s) = \alpha''(s)$). Assume that $(T(s),N(s))$ is a positive basis of the plane they span (for example, assume that the Euclidean cross product $T(s)\times_E N(s)$ is future-directed).

Say I now complete that frame with a lightlike "binormal vector" $B(s)$ orthogonal to $T(s)$ that makes the basis $(T(s),N(s),B(s))$ positive (with normalization constraint $\langle N(s),B(s)\rangle_L = -1$). We have a Frenet-like system $$ \begin{pmatrix} T'(s) \\ N'(s) \\ B'(s)\end{pmatrix} = \begin{pmatrix} 0 & 1 & 0 \\ 0 & \tau(s) & 0 \\ 1 & 0 & -\tau(s)\end{pmatrix}\begin{pmatrix} T(s) \\ N(s) \\ B(s) \end{pmatrix},$$where $\tau(s)$ is the so-called pseudo-torsion of $\alpha$.

Assuming $\alpha(0) = 0$ and calling $\mathcal{F} = (T(0),N(0),B(0))$, Taylor expansion gives $$\alpha(s) - R(s) = \left(s, \frac{s^2}{2} + \tau(0) \frac{s^3}{6},0 \right)_{\mathcal{F}},$$for some $R(s)$ with $R(s)/s^3 \to 0$. This in particular hints that every curve in these conditions is a plane curve (this is actually true, not that hard to prove).

Projecting that in the osculating plane of the curve, we can consider $\gamma\colon I \to \Bbb R^2$ given by $\gamma(s) = (s, \frac{s^2}{2} + \tau(0) \frac{s^3}{6})$, where the metric in this $\Bbb R^2$ is only ${\rm d}x^2$.

I'd like to know if there is any reasonable notion of curvature for $\gamma$ related to the pseudo-torsion $\tau(0)$. Naively I could compute $$\kappa_\gamma(s) = \frac{\det(\gamma'(s),\gamma''(s))}{\|\gamma'(s)\|^3} = 1+\tau(0)s,$$but this is far from satisfactory for me.