The Sum-Product Algorithm for Degree-2 Check Nodes and Trapping Sets
  The sum-product algorithm for decoding of binary codes is analyzed for bipartite graphs in which the check nodes all have degree $2$. The algorithm simplifies dramatically and may be expressed using linear algebra. Exact results about the convergence of the algorithm are derived and applied to trapping sets. 
John O. Brevik, Michael E. O'Sullivan
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