Those stellar motions can reveal the presence of unseen planets whose gravitational influence produces periodic patterns of motion in the host star's position. (scientificamerican.com)
Even more popular than this idea of particular causes for particular deviations was an assumption that features of nature follow periodic patterns, diverging only to return. (history.aip.org)
The optical lattice was generated using two laser beams traveling in opposite directions, whose fields add up to form a sinusoidal periodic pattern in one dimension. (sciencedaily.com)