HypergeometricU[a, b, z] is the confluent hypergeometric function .
Mathematical function (see Section A.3.10).
The function has the integral representation .
HypergeometricU[a, b, z] has a branch cut discontinuity in the complex z plane running from to .
See The Mathematica Book: Section 3.2.10.
See also: Hypergeometric1F1, Hypergeometric0F1.