This figure depicts two complex sinusoids, colored gold and cyan, that fit the same sets of real and imaginary sample points. They are thus aliases of each other when sampled at the rate (fs) indicated by the grid lines. The gold-colored function depicts a positive frequency, because its real part (the cos function) leads its imaginary part by 1/4 of one cycle. The cyan function depicts a negative frequency, because its real part lags the imaginary part.
<a href="//commons.wikimedia.org/w/index.php?title=User:Jarvisa&amp;action=edit&amp;redlink=1" class="new" title="User:Jarvisa (page does not exist)">Andrew Jarvis</a>
2019-06-05
<span class="int-own-work" lang="en">Own work</span>