Feng Qi, Qing Zou, Bai-Ni Guo, THE INVERSE OF A TRIANGULAR MATRIX AND SEVERAL IDENTITIES OF THE CATALAN NUMBERS, Applicable Analysis and Discrete Mathematics, Vol. 13, No. 2 (October 2019), pp.

The R matrix is special because it is upper triangular -- the values below the diagonal are 0s -- and its inverse is easy to compute. To compute the inverse of a matrix A = Q * R, the math derivation ...

Mohamed Barraa, Mohamed Boumazgour, A Note on the Spectrum of an Upper Triangular Operator Matrix, Proceedings of the American Mathematical Society, Vol. 131, No. 10 (Oct., 2003), pp. 3083-3088 ...