I'm writing a paper and I use a list to describe some notation in an equation:

blah blah is described by the equation
where x is the state vector, u (is) the input vector, and d (are) the disturbances.

Do I need the "is" and "are" which I put in parentheses? My gut feeling is that I can skip them, but I have a hard time convincing myself.

Edit: I made a small notation edit and added the word "vector" in two places. If this isn't consistent with what you want to write, you can roll the edit back to your original.

  • 2
    It is usual in technical writing not to omit any words in order to keep the meaning as clear as possible.
    – Mick
    Commented Nov 10, 2016 at 13:57
  • 'd' has to be a single independent variable. Should this be '...d1, d2 ... are the disturbances'? // x=f(x,u,d) seems to confuse an independent and the dependent variable. Commented Nov 10, 2016 at 15:24
  • @EdwinAshworth Think of x,u,d as vectors. They each represent a subset of the state vector. In an aircraft, x are related to vehicle geometry and velocity, u describe control surface variables and throttle, and d could represent a disturbance of interest, such as a downdraft. The subdivisions aren't fixed. The same acceleration might be a velocity disturbance in one analysis, but an acceleration held in x in another. This is pretty normal linear analysis notation. The disturbances d are usually differentials to values carried in x.
    – Phil Sweet
    Commented Dec 10, 2016 at 16:17
  • @EdwinAshworth Sorry for the above, I only just noticed the duplication of "x". Yes, some other variable should be substituted.
    – Phil Sweet
    Commented Dec 10, 2016 at 16:34

1 Answer 1


when writing the paper with some equation and parameters, we have many ways to describe the parameter in your equation. For example, x=f(x,u,d), where x represents the state, let u and d be the input and the disturbances, respectively.

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