Tomno, Victor, et al. “The Wss P0 Matrix Completion Problem for Symmetric Patterns of Acyclic Digraphs of Order Four”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 37, no. 1, Jan. 2018, pp. 112-21, https://www.gssrr.org/JournalOfBasicAndApplied/article/view/8591.