A professor from the University of Pennsylvania was the center of a flight delay last week after another passenger reported him for suspicious behavior – for working on a mathematics problem.

Guido Menzio, an associate professor of economics at the Ivy League school, was bound for Syracuse on an American Airlines flight on Thursday. After boarding the airplane and finding his seat, Dr. Menzio was solving a math equation when a woman sitting next to him reported his apparently cryptic writing to a flight attendant. Menzio said the plane then returned to the gate before he was questioned by the pilot and an official.

“[T]hey tell me that the woman was concerned that I was a terrorist because I was writing strange things on a pad of paper,” he told the Associated Press.

The Washington Post reported that the woman informed the flight crew that she was ill, but after disembarking told them of her worry. American Airlines spokesman Casey Norton said that security officials then spoke with Menzio, who is Italian, and concluded that he was not a “credible threat.”

What Menzio’s fellow passenger may have thought to have been a secret terrorist code was in fact a differential equation the professor was preparing for a talk that he had been scheduled to give at Queen’s University.

Developed in the 17th century independently by Isaac Newton and Gottfried Leibniz, differentials are calculous equations that relate functions, that is, predefined relations between inputs and outputs, to their derivatives, that is, rates of change. In biology, they can be used to determine population growth, in chemistry to model reactions, in physics to study waves, and, in economics to find the effects of changing prices, to name a few examples.

Menzio's equation concerned the effects of "menu costs," that is the cost of a seller changing its prices, on pricing behavior, and likely contained mathematical symbols unfamiliar to those who haven't studied calculus.

“There was definitely a conversation about the concerns that [Menzio's aisle-mate] raised,” Mr. Norton told Time. “Anytime a customer expresses concerns we are going to look into it and determine that the flight is 100 percent safe to fly.”

Norton also told the Post that, in the event of disputes between passengers, American tries “to work with them peacefully to resolve it.”

For his part Menzio said he was treated respectfully but is bothered by the possible sentiments behind his neighbor’s suspicions. Menzio said that a “broken system that does not collect information efficiently,” should not be triggered by “people who may be completely clueless,” according to the Post.

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Menzio’s treatment echoes other recent incidents involving airline passengers being delayed or removed from flights due to other passengers’ suspicions or inclinations. Last month, a University of California, Berkeley, student was removed from his flight after another passenger was upset over his use of Arabic in a cell phone conversation. Earlier in April, a Maryland woman said she was removed from her flight because she was wearing a hijab.

“Not seeking additional information after reports of ‘suspicious activity’ ... is going to create a lot of problems, especially as xenophobic attitudes may be emerging,” Menzio told the AP.