Relating to or functioning as a determining factor; in mathematics, relating to a determinant (a special number calculated from a square array of numbers).
From determinant + -al (adjective suffix meaning 'of or relating to'). In mathematics, determinant comes from Latin determinans, present participle of determinare, because these numbers 'determine' solutions to systems of equations.
Determinantal matrices are like mathematical Swiss Army knives—that single number (the determinant) tells you whether a system of equations has a unique solution, which is mind-blowingly useful in engineering and physics.
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