We already saw a square (one of plane figures) is given by

with absolute function |*x*|.
There is an another equation using which is given by

in other work of mine (see ``plane figure equations''). This provides a resort to study discontinuous functions with linear calculus.

The expedient is as follows.

- Suppose the problem in question contains discontinuous functions.
- Express discontinuous functions as a continuous functions
with parameters (as
*n*in equation (2)) so as to approach the discontinuous functions under the limit of the parameter. We call them pseudo functions. - Study the problem with the continuous pseudo functions instead of the discontinuous functions.
- Take a limit of introduced parameters in the results of pseudo functions.