through determinant expansion techniques, series ex- pansions, spanning tree enumerations, etc. There is no predefined template for the expressions that result, i.e. the form of the expressions is developed at run time. An alternative technique that was recently proposed builds on the existence of a pre- defined and parametrized template expression [Dae03]. Using numerical simulation and data regression, one tries to determine suitable values for the template parameters. This method suits very well for performance and strongly nonlinear system modeling. 4.3.2 Symbolic LPTV system analysis: outlining the flow Basically, they can model anything that can be simulated. The disadvantage, of course, lies in the need to find a suitable expression template a priori. 4.3.1.4 Contributions of this work This section presents an algorithm to construct symbolic expressions for the HTM el- ements of LPTV systems. The method is called SymbolicHTM. The input is a system model in the form of a block diagram. The output consists of symbolic expressions for the system's HTM-elements in terms of the parameters characterizing the behavior of its building blocks. The algorithm is based on analytical methods. There is no need for a predefined expression template. The input block diagram can, for example, be extracted from a circuit netlist using techniques similar to the ones for behavioral modeling of LTI networks [Leyn01]. For circuits consisting of a large number of nodes, more compact expressions are obtained by modeling the often large LTI subcircuits using multiport representations. With Sym- bolicHTM expressing the results in terms of symbols representing the multiport para- meters, lengthy and uninterpretable expressions are avoided. The symbolic computa- tion of these multiport parameter characteristics is then accomplished afterwards, using well-established techniques for (LTI) symbolic circuit analysis [Fern98, Gie91, Lin91]. Hence, In the hierarchy of symbolic analysis, SymbolicHTM makes up a layer on top of the existing to