Differences between Two Notations in DE

[lkh1986]

Homework Statement

Is there any difference between the two notations below? Or can they be used interchangeably?

$$\tilde{x}$$ and $$\underline{x}$$ (not an underline, but the tilde is at the bottom of the unknown x)

Homework Equations

The Attempt at a Solution

From my observation, $$\underline{x}$$ is mostly used to denote a row matrix, or a vector, while $$\tilde{x}$$ is used to denote a vector function, is it correct? Thanks.

 

[HallsofIvy]

Those are NOT standard notations so can be used any way you want as long as you define them when you do.