Scrabble?! TRIDI

Definitions of TRIDI in various dictionaries:

In linear algebra, a tridiagonal matrix is a band matrix that has nonzero elements only on the main diagonal, the first diagonal below this, and the first diagonal above the main diagonal. For example, the following matrix is tridiagonal: ( 1 4 0 0 3 4 1 0 0 2 3 4 0 0 1 3 ) . {\displaystyle {\begin{pmatrix}1&4&0&0\\3&4&1&0\\0&2&3&4\\0&0&1&3\\\end{pmatrix}}.} The determinant of a tridiagonal matrix is given by the continuant of its elements.An orthogonal transformation of a symmetric (or Hermitian) matrix to tridiagonal form can be done with the Lanczos algorithm.