DT: Soviet Union: double taxation agreement, Article 15 Income of teachers and researchers
(1) Notwithstanding the provisions of Article 13 remuneration for teaching or research derived by an individual who visits one of the Contracting States for a period not exceeding three years for the purpose of teaching or engaging in research at a university or other educational institution (including in the case of the USSR a research institution) in that State shall be exempted from tax in that State provided:
(a) he was a resident of the other Contracting State immediately before that visit; and
(b) the exemption will not exceed a period of three years from the date he first visits the first-mentioned State for such purposes.
(2) The provisions of paragraph (1) of this Article shall not apply to income from research undertaken primarily for the benefit of private persons.