@article{MorNo01,
 author         = {J. L. Morales and J. Nocedal},
 title          = {Algorithm PREQN: FORTRAN Subroutines for Preconditioning
                   the Conjugate Gradient Method},
 journal        = SIOPT,
 volume         = 27, number = 1, pages = {83-91}, year = 2001}