EXPM_BLOCK_TRI - Exponential of block triangular matrix.
[Fa,Fb,L] = expm_block_tri(A,B,E) computes Fa = e^A, Fb = e^B, and L = D_{exp}(A,B,E), the (1,2) block of exp([A E;O B]), without explicitly computing the exponential of the block triangular matrix, exp([A E;O B]).
References: A. H. Al-Mohy, Generalizing the Fr'echet Derivative Algorithm for the Matrix Exponential, Submitted.
A. H. Al-Mohy and N. J. Higham, Computing the Fr'echet Derivative of the Matrix Exponential, with an Application to Condition Number Estimation, SIAM J. Matrix Anal. Appl., 30(4), (2009), pp. 1639-1657.
A. H. Al-Mohy and N. J. Higham, A new scaling and squaring algorithm for the matrix exponential, SIAM J. Matrix Anal. Appl., 31(3), (2009), pp. 970-989.
Note: qtri_struct used by expm, a MATLAB internal function, ...\toolbox\matlab\matfun\private\qtri_struct