subdiagonal

From sub- +‎ diagonal.

subdiagonal (not comparable)

1. (mathematics) Said of an entry of a square matrix: that it lies anywhere in the lower triangular area below the main diagonal of the matrix.
   • 2010, Martin Aigner with Günter M. Ziegler, chapter 16, in Proofs from THE BOOK, 4th edition, Delhi, India: Springer (India), published 2013, page 97:

     (3) From R, we obtain the set of points ${\displaystyle \mathbb {R} ^{\tbinom {n}{2}}}$ whose coordinates are the subdiagonal entries of the corresponding matrices:
     ${\displaystyle S:=\{(\mathbf {x} \mathbf {x} ^{T})_{i>j}:\mathbf {x} \mathbf {x} ^{T}\in R\}}$.

subdiagonal (plural subdiagonals)

1. (mathematics) The diagonal of a matrix that lies directly below and to the left of the main diagonal.