@article{AL_2012_51_4_a0,
     author = {J. Wallbaum},
     title = {A~$\Delta^0_2$-poset with no positive presentation},
     journal = {Algebra i logika},
     pages = {423--428},
     publisher = {mathdoc},
     volume = {51},
     number = {4},
     year = {2012},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_2012_51_4_a0/}
}