@article{ZNSL_2007_348_a4,
     author = {S. Nicaise and S. I. Repin},
     title = {Functional a posteriori error estimates for the
},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {127--146},
     publisher = {mathdoc},
     volume = {348},
     year = {2007},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2007_348_a4/}
}