I have just wasted about three hours of sunday afternoon trying to compute a modified bessel function of second kind of fractional order (also known as hyperbolic Bessel function). I hope that this tiny C++ snippet will be googlable, and it will save a few minutes of someone’s live:

#include <boost/math/special_functions/bessel.hpp>


double v = 5.0/3.0; /* an order of Bessel function */
double x = 2.5; /* an argument of bessel function */
double r; /* will contain a value */

r = boost::math::cyl_bessel_k(v, x);

Just in case someone is interested what is a modified Bessel function, this is an integral representation of Bessel funtion of order v:

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