The letter “x” often symbolizes something unknown, with an air of mystery that can be appealing. Illustration by Meilan Solly Even though “x” is one of the least-used letters in the English alphabet, ...

Peter Schumer does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond ...

Mathematicians predicted that if they imposed enough restrictions on how a shape might tile space, they could force a periodic pattern to emerge. But they were wrong. One of the oldest and simplest ...