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There is a mathematical explanation for the auditory illusion in my example, sometimes called the missing fundamental: The perceived pitch is the greatest common divisor of the frequencies of the sine waves present.

The reader probably never has had occasion to compute a greatest common divisor since he left school.

The greatest common divisor of all the minorities that go to making a winning national combination must be neutral, he must be colorless, he must not know that his soul is his own.

The videos were each about 10 minutes long and contained two elements: his blackboard-style diagrams—Khan happens to be an excellent sketcher—and his voice-over explaining things like greatest common divisors and equivalent fractions.

What'll he do wid the greatest common divisor an' the indicative mood an' the Sea of Azov, an' the Zambezi River, when he's learned 'em, anyhow?