Aktive Filter Design Handbook (Bulgarian)

[edryer]

Fortunately, Russian is my second language ))))

This is interesting, as of 1981 in the USSR as was, the TI-59 would have been an extreme rarity (likely the number in the country counted on one hand) but hardware clones were always around (there was even a PDP-11 architecture based Calculator, Elektronika MK-85 and the beautiful MK-90, based upon RT-11!!!, so a TI-59 would have been trivial, just a few years or so before these models).

So, the TI-59 must have had a Soviet clone, as well, 1981 = 3-4 years of time to clone. cannot find such a machine though!

My guess an early Elektronika MK model, before the Elektronika MK-61.

PS. The document though does say TI Programmable (in Russian).