Argument, x >= 0 (NaN for negative x).
Order, any real number (K(x, -nu) = K(x, nu)).
exponScaled (see BesselOptions).
K_nu(x), or exp(x) K_nu(x) when exponScaled.
For orders above 1000, results far out of the range of doubles are returned at once (Infinity or 0, decided by
bounds on their log); otherwise the time grows with the order (a recurrence of about nu steps): several seconds at
nu = 1e8. Beyond 2^28 steps, and for besselI where its continued fraction does not converge (x above about
5e10), Debye's uniform expansion is used; a RangeError is thrown only where its exponent nu eta - x is formed from
terms above 2^26 and the result is in range (near x = 0.66 nu, unscaled).
Modified Bessel function of the second kind. Equivalent to R's
besselK(x, nu, expon.scaled = FALSE). Takes single numbers.