— Function File: starp (P, K, ny, nu)

Redheffer star product or upper/lower LFT, respectively.

          
          +-------+
          --------->|       |--------->
          |   P   |
          +--->|       |---+  ny
          |    +-------+   |
          +-------------------+
          |  |
          +----------------+  |
          |                   |
          |    +-------+      |
          +--->|       |------+ nu
          |   K   |
          --------->|       |--------->
          +-------+

If ny and nu “consume” all inputs and outputs of K then the result is a lower fractional transformation. If ny and nu “consume” all inputs and outputs of P then the result is an upper fractional transformation.

ny and/or nu may be negative (i.e. negative feedback).