Solved
@Cornel wrote:
And the only solution for this is this (to define Tf above f2)?
NO!!
You simply have to use your function the way you had defined it - with two arguments!
As i already wrote, you were missing the second argument when you called your own function.

Here a an alternative function

Calling the "mod" function twice is necessary to deal correctly with negative t-values because the implemented "mod" function would return negative values for negative arguments.
Another alternative:

could also written as

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