An economist was taken off a US flight and questioned by officials after a fellow passenger spotted him working on a mathematical equation.

Guido Menzio, an Italian maths professor, was removed from the Philadelphia to Syracuse flight after the woman sitting next to him told cabin crew that she was suspicious of the differential equation he was working on.

Mr Menzio showed agents what he had been writing and the flight eventually took off more than two hours late.

The University of Pennsylvania associate professor wrote on Facebook that the experience was "unbelievable" and made him laugh.

"The lady just looked at me, looked at my writing of mysterious formulae, and concluded I was up to no good," he said.

"Because of that an entire flight was delayed."

Mr Menzio added that not seeking additional information after reports of suspicious activity "is going to create a lot of problems, especially as xenophobic attitudes may be emerging".

American Airlines, whose regional partner Air Wisconsin was operating the flight, said the crew followed protocol to investigate a passenger's concerns.