Geodesic
The number of antichains in ranked partially ordered sets
Diskretnaya Matematika, Tome 1 (1989) no. 1, pp. 74-93 
A. A. Sapozhenko. The number of antichains in ranked partially ordered sets. Diskretnaya Matematika, Tome 1 (1989) no. 1, pp. 74-93. http://geodesic.mathdoc.fr/item/DM_1989_1_1_a7/
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     author = {A. A. Sapozhenko},
     title = {The number of antichains in ranked partially ordered sets},
     journal = {Diskretnaya Matematika},
     pages = {74--93},
     year = {1989},
     volume = {1},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DM_1989_1_1_a7/}
}
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We obtain the asymptotic behavior of the number of antichains in partially ordered sets whose diagrams are bipartite graphs that possess extension properties and whose number of vertices does not exceed $c\log_2\kappa$, where $\kappa$ is the minimum of the degrees of the vertices and $c$ is a constant less than 3. As a consequence we obtain the well-known asymptotic behavior of the number of binary codes with distance 2.