# Proving set of register machines that halt before k steps for some input is non-recursive

Given an enumeration of register machines $$R_n$$ that take a single natural number as input, and a constant $$k$$, the function $$f$$ is defined as:

$$f(n) = \begin{cases} 1 & \exists m \text{ such that } R_n(m) \text{ halts in } k \text{ steps} \\ 0 & \text{otherwise} \end{cases}$$

I'm trying to show that this function is not recursive, but am unsure of how to go about doing so? If anybody could give me any advice I'd be very grateful.