@article{MS2_1978__43_166627,
     author = {Urban Cegrell},
     title = {Delta-plusharmonic functions.},
     journal = {Mathematica Scandinavica},
     pages = {343--352},
     year = {1978},
     volume = {43},
     zbl = {0393.31006},
     url = {http://geodesic.mathdoc.fr/item/MS2_1978__43_166627/}
}