The lasso is used to determine which edges are present in a topic graph. The original R code was written by David M. Blei and coauthors and is available together with the C code for fitting the CTM.
1  build_graph(x, lambda, and = TRUE)

x 
Object of class 
lambda 
Numeric in [0,1] indicating the relative bound on the L1norm of the parameters. 
and 
Logical; if 
Returns an adjacency matrix for the topics versus topics graph.
David M. Blei, modified by Bettina Gruen.
Blei D.M., Lafferty J.D. (2007). A Correlated Topic Model of Science. The Annals of Applied Statistics, 1(1), 17–35.
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