\varkappaThe symbol \varkappa is a variant of the Greek letter kappa (κ). It is used in various mathematical contexts, often in set theory or to denote a specific kind of curvature in differential geometry.
Curvature of a circle with radius R
\varkappa = \frac{1}{R}Complement of set A in set theory
\varkappa(A) = \{ x \mid x \notin A \}Integral involving an exponential decay with rate \varkappa
\int_0^\infty e^{-\varkappa x} \, dx