下した三さん角かく行ぎょう列れつ

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Meanings

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noun

lower triangular matrix

mathematics

A square matrix where all entries above the main diagonal are zero. Used in linear algebra.

上うえ三さん角かく行ぎょう列れつ
上三角行列 is an upper triangular matrix, where entries below the main diagonal are zero, while 下三角行列 has zeros above the diagonal.
三さん角かく行ぎょう列れつ
三角行列 is a general triangular matrix; 下三角行列 specifically refers to the lower triangular case.

Compound of 下 (lower) + 三角 (triangle) + 行列 (matrix). A straightforward technical term from linear algebra.