BesselK[n, z] gives the modified Bessel function of the second kind .
Mathematical function (see Section A.3.10).
satisfies the differential equation .
BesselK[n, z] has a branch cut discontinuity in the complex z plane running from to .
FullSimplify and FunctionExpand include transformation rules for BesselK.
See The Mathematica Book: Section 3.2.10.
See also: BesselI, AiryAi, BesselJ.