The LaTeX command for varkappa (ϰ) is \varkappa (variant kappa).
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Write \varkappa inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amssymb package, so add \usepackage{amssymb} to your preamble.
Greek letters work in math mode only: in running text write $\varkappa$, or load the textgreek package for upright text forms. With unicode-math under XeLaTeX or LuaLaTeX, typing ϰ in a formula gives the same symbol.
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Lowercase Greek letters in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \varkappa in math mode after adding \usepackage{amssymb} to your preamble. For example, $\varkappa \in \mathbb{R}$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.