@article{10_4171_dm_269,
     author = {Daniel Lenz and Peter Stollmann and Ivan Veseli\'c},
     title = {The {Allegretto-Piepenbrink} theorem for strongly local {Dirichlet} forms},
     journal = {Documenta mathematica},
     pages = {167--189},
     year = {2009},
     volume = {14},
     doi = {10.4171/dm/269},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/dm/269/}
}