Alexandr V. Zelenin, Dmitry V. Rudnev. Copular verb slyt’: the history of modalization
RJANO. 2018. № 1 (35), Pages. 85 - 109

Dmitry V. Rudnev. On the origin of the copular verb stat´
RJANO. 2014. № 2 (28), Pages. 82 - 103