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Section: Mathematical Functions
Computes the inverse hyperbolic secant of its argument. The general syntax for its use is
y = asech(x)
 where x is an n-dimensional array of numerical type. 
The asech function is computed from the formula 
![\[ \mathrm{sech}^{-1}(x) = \cosh^{-1}\left(\frac{1}{x}\right) \]](form_49.png) 
Here is a simple plot of the inverse hyperbolic secant function
--> x1 = -20:.01:-1;
--> x2 = 1:.01:20;
--> plot(x1,imag(asech(x1)),x2,imag(asech(x2))); grid('on');
