But for action pertaining to fields, it may be integrated over spatial variables as well.

If you made all the spatial variables constant, the spacetime functions reduced to pure time; so you could transform energy to time or vice versa.

The interval length is not required to be the same for each spatial variable .

This momentum operator is in position space because the partial derivatives were taken with respect to the spatial variables.

The scalar field, is a function of a set of spatial variables, , and a time variable, t. We first consider the linear theory.

The classic bifurcation analysis is referred to as an ordinary differential equation system, which is independent on the spatial variables.

Likewise, one can make the surface height itself be a continuous function of the spatial variables.

A spatial model is one that has one or more state variables that are a function of space, or can be related to other spatial variables.

A picture function is a mathematical representation of a two-dimensional image as a function of two spatial variables.

By assuming that light intensities remain constant within a short exposure period, the field can be modeled as only a function of the spatial variable .