@article{10_4064_aa136_2_5,
     author = {Alan Filipin},
     title = {An irregular $D(4)$-quadruple cannot be
 extended to a quintuple},
     journal = {Acta Arithmetica},
     pages = {167--176},
     publisher = {mathdoc},
     volume = {136},
     number = {2},
     year = {2009},
     doi = {10.4064/aa136-2-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/aa136-2-5/}
}