Wall-Sun-Sun prime
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In mathematics, a Wall-Sun-Sun prime is a certain kind of prime number. A prime p > 5 is called a Wall-Sun-Sun prime if p² divides
where F(n) is the nth Fibonacci number and 
is the Legendre symbol of a and b.
Wall-Sun-Sun primes are named after D. D. Wall, Zhi Hong Sun and Zhi Wei Sun; Z. H. Sun and Z. W. Sun showed in 1992 that if the first case of Fermat's last theorem was false for a certain prime p, then p would have to be a Wall-Sun-Sun prime. As a result, prior to Andrew Wiles' proof of Fermat's last theorem, the search for Wall-Sun-Sun primes was also the search for a counterexample to this century-old conjecture.
No Wall-Sun-Sun primes are known to date; if any exist, they must be > 1014. It has been conjectured that there are infinitely many Wall-Sun-Sun primes, but the conjecture remains unproven.
