The work of E. Vikulov is connected to the new field of scientific research that combines signal processing, algorithm complexity and optimal estimation theory. E. Vikulov correctly interpreted the problem of echo canceller for a long regular echo. He reduced the application problem to the subject of his work, that is, to the shortened autoregression model. The most difficult was to compare  a direct and a shortened model. This has been done in Theorem 4. E. Vikulov also testeda  number of results by computer modeling.  E. Vikulov has shown himself as a careful specialist that is capable to conduct his own scientific researches. The results of the work will be reported at the International conference «Speech and Computer 2017» and will be published in a periodical  of Springer. I think the work is worth the excellent mark.