X
It had come to him as a flash of lightning, a rainy evening with far too much homework to do. "X is the answer to all equations". Javier had felt as if he had caught a glimpse of the engines behind the universe, as if a small window in the sky had opened to reveal a glimmering light before closing again.
That rainy night in Salamanca was the defining moment of his life, it was the reason why he now, twenty years later, found himself lecturing at a prestigious university in the US, the reason why his name tag said Xavier Morano, PhD Number theory.
Xavier added some notes to his article on Hilbert's tenth problem, and leaned back in his comfortable chair. Absentmindedly he rolled up his sleeve, and looked at a clumsily tatooed X on his biceps. He had done it himself that night, unable to sleep, struck by the purity of his vision. He could still taste the disappointment from the day after, when his teacher had casually mentioned that the unknown factor could be any letter.
His career in mathematics had been a search for that clarity again, that boundless joy. He could just as well have set off on a search for the holy Grail.