ldl_blocktoep — LDL factorization of a strongly regular hermitian block Toeplitz matrix with positive-definite first block
[L,d] = ldl_blocktoep(BC)
The routine computes the LDL factorization of a hermitian but possibly indefinite block Toeplitz matrix T=[T(0) T(1)' T(2)' ... T(M-1)'; T(1) T(0) T(1)' ... T(M-2)'; ... ;T(M-1) T(M-2) T(M-3) ... T(0)] with first block column BC=[T(0);T(1);...;T(M-1)]. Here, the blocks T(0),...,T(M-1) are r x r matrices. We assume that the first block T(0)=T(0)'=BC(1:r,1:r) is hermitian and positive-definite and that the matrix T is strongly regular (i.e., all leading minors are non-zero). The algorithm computes a lower triangular matrix L and a vector d with entries +1 / -1 such that T=L*diag(d)*L'.