Investigating Test Functions: \psi(x) & \phi(x2)

[squenshl]

Homework Statement

Investigate whether \psi(x) = \phi(c(x - \eta)) & \psi(x) = \phi(x²) are test functions.

Homework Equations

The Attempt at a Solution

The first function is smooth but has no contact support as it is only 0 at x = \eta so this is not a test function.
The second function is smooth but is not 0 at any interval, so this is not a test function.

 

[arkajad]

But you did not say what are the assumptions about \phi. Without knowing them one can't answer these questions.

 

[squenshl]

Sorry, assume that \phi(x) is a test function on (-\infty,\infty)

 

[squenshl]

They are both test functions I think.

 

[arkajad]

You see, there are many kinds of test function spaces. You probably have learned about just one. To see that some function is a test function in the sense you know it, you need to check the precise definition of your test function space. Which conditions a given function must satisfy to be a test function? Differentiable? How many times? Compact support? Or vanishing sufficiently fast at infinity?