Are the following frequency domain signals valid DTFT's (Discrete Time Fourier Transforms)? Answer yes or no, and justify your answer.
a) $X(\omega) = \omega + \pi$
b) $X(\omega) = i + \pi$
c) $X(\omega) = \sin(10\omega)$
d) $X(\omega) = \sin(\omega/10)$
I'm just very unsure as to how to go about this question (what methodology to use). Could anyone possibly point me in the right direction? Thanks!