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Gap metric and Vinnicombe (nu-gap) metric for distance between two systems

`[`

computes the gap and Vinnicombe (`gap`

,`nugap`

] = gapmetric(`P1,P2`

)*ν*-gap) metrics for the distance between
dynamic systems `P1`

and `P2`

. The gap metric values
satisfy 0 ≤ `nugap`

≤ `gap`

≤ 1. Values close to zero imply that any controller that stabilizes
`P1`

also stabilizes `P2`

with similar closed-loop
gains.

[1] Zhou, K., Doyle, J.C.,
*Essentials of Robust Control*. London, UK: Pearson, 1997.