 He also conjectured that all even perfect numbers come from Euclid's construction using Mersenne primes, but was unable to prove it.Another Islamic mathematician, Ibn al-Banna' al-Marrakushi, observed that the sieve of Eratosthenes can be sped up by testing only the divisors up to the square root of the largest number to be tested.Fibonacci brought the innovations from Islamic mathematics back to Europe.