The LaTeX command for det is \det (determinant).
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Write \det inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
\det sets the name upright with operator spacing; typed as plain letters it would print in italics, as a product of variables.
$\det_{i=1}^{n} x_i$Open this example in the live preview: it renders as you type, with no compiler and no signup.
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Use \det in math mode; it needs no extra package. For example, $\det_{i=1}^{n} x_i$.
None: \det is built into LaTeX, so no extra package is required.