# Determining Valid DTFT's

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!

• Shouldn't a valid DTFT be $2\pi$-periodic? – Matt L. Mar 17 '14 at 8:27
• Maybe consulting a table of DTFTs will give you some ideas as to what typical DTFTs look like. – Dilip Sarwate Mar 17 '14 at 12:42