Sergey Kitaev, Jeffrey B. Remmel. Enumerating (2+2)-free posets by the number of minimal elements and other statistics. Discrete Applied Mathematics, 159(17):2098-2108, 2011. [doi]
@article{KitaevR11,
title = {Enumerating (2+2)-free posets by the number of minimal elements and other statistics},
author = {Sergey Kitaev and Jeffrey B. Remmel},
year = {2011},
doi = {10.1016/j.dam.2011.07.010},
url = {http://dx.doi.org/10.1016/j.dam.2011.07.010},
researchr = {https://researchr.org/publication/KitaevR11},
cites = {0},
citedby = {0},
journal = {Discrete Applied Mathematics},
volume = {159},
number = {17},
pages = {2098-2108},
}