Math symbol in German thesis from 1963
Solution 1:
Note the "C" (third character in third row) and the "s" (fourth character in second row) above.
The important thing is Daniel Fischer's comment from above that "$\operatorname{Cos}$" here does not mean $\cos$ but rather $\cosh$.
Solution 2:
To guess, except x everything else is a constant.
Left hand side of entire last line is proportional to 1/$\sqrt(1- x)$. So, if
x < 1 the first line is $ cosh( \alpha/3) $ for real argument and, if
x > 1 then first line should be taken to mean $ cos( \alpha/3) $ for imaginary argument
since $ cosh (i \alpha) = cos ( \alpha) $.