Two adjacent diagonal elements or blocks could not be successfully exchanged. This error can only occur if the exchange involves at least one
2 by
2 block; it implies that the problem is very ill-conditioned, and that the eigenvalues of the two blocks are very close. On exit,
T may have been partially reordered, and
ILST points to the first row of the current position of the block being moved;
Q (if requested) is updated consistently with
T.