@inbook {700, title = {Stable estimates in equation-error identification: an open problem}, booktitle = {Open Problems in Mathematical Systems and Control Theory}, year = {1999}, publisher = {Springer-Verlag}, organization = {Springer-Verlag}, chapter = {28}, isbn = { 1-85233-044-9}, url = {http://perso.uclouvain.be/vincent.blondel/books/openprobs/}, author = {R. L{\'o}pez-Valcarce and S. Dasgupta} }