Examination at the Faculty of Mathematics of <a href="https://en.wikipedia.org/wiki/Petrozavodsk_State_University" class="extiw" title="w:Petrozavodsk State University">Petrozavodsk State University</a>.
Vladimir Grigoryev
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Personal archive of Vladimir Grigoryev: <a rel="nofollow" class="external text" href="http://grigoriev.gallery.ru/watch?a=bnDk-mAcm">gallery.ru</a>.