Hofmann, Karl H. and Ruppert, (1997):
Lie groups and subsemigroups with surjective exponential function.
Providence, RI: American Math. Soc., 1997. VIII, 174 S,
Providence, RI, American Math. Soc, 618, [Book]

Krozer, Viktor and Ruppert, and Schüßler, and Fricke, and Lee, and Hartmann, (1996):
Calculation of the power capabilities of HBT amplifiers based on a new physical HBT model.
In: International journal of microwave and millimeter-wave computer-aided engineering. 6 (1996), No. 4, S. 270-280, [Article]

Hofmann, Karl H. and Ruppert, (1995):
Lie groups and exponential Lie subsemigroups.
In: Semigroups in algebra, geometry and analysis. Hrsg.: K.H. Hofmann u.a. - S. 159-198, Berlin, New York, de Gruyter, [Book Section]