More than 350 years ago, a French mathematician wrote a deceptively simple theorem in the margins of a book, adding that he had discovered a marvelous proof of it but lacked space to include it in the margin.

He died without ever offering his proof, and mathematicians have been trying ever since to supply it. Now, after thousands of claims of success that proved untrue, mathematicians say the ++ daunting challenge, perhaps the most famous of unsolved mathematical problems, has at last been surmounted.

The problem is known as Fermat's last theorem, and its apparent conqueror is Dr. Andrew Wiles, 40, an English mathematician who works at Princeton University. Dr. Wiles announced the result yesterday at the last of three lectures given over three days at Cambridge University in England.