] this implementation follows the convention
          used by Mathematica: the function is periodic with period &#960; &#160; in &#952; &#160; and 2&#960; &#160; in &#966;.
          Please note that this is not the behaviour one would get from a casual
          application of the function's definition. Cautious users should keep &#952; &#160; and
          &#966; &#160; to the range [0, &#960;] and [0, 2&#960;] respectively.
        </p>
<p>
          See: <a href="http://mathworld.wolfram.com/SphericalH