Hello, leigh!

Where is the following derivative defined?

. .

For clarity, I wrote the function like this:

Clearly the derivative is definedwithinthose intervals.

The only doubts occur at the endpoints.

Test an endpoint using one-sided limits.

. .

. .

The limit does not exist at

Hence, the derivative is not defined at

It can be shown that the derivative is undefined for any odd multiple of

is defined for all except at for any integer