TransliterationGrigory Gelbak. Elementary Calculation of the Side of the Ninth Tangle. P.P. Soikin. 1925. In Russian (ask us if in doubt)
In EnglishGrigoriy Gel'bak. Elementarneyshee vychislenie storony devyatiugol'nika. Izd. P.P. Soykina. 1925 g.
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