Answer: Leonhard Euler.
Swiss-born Euler is held to be one of the greatest mathematicians in history. He made pioneering contributions to several branches of mathematics and introduced much of the modern mathematical terminology and notation.
Answer: Pierre-Simon Laplace has an Erdős number of at most 14!
According to "The Erdös number project" a path to Laplace was found by Leonid Yanushevich giving the famous mathematician an Erdös number of at most 14.
Answer: A perfect number.
You might also think of a perfect number as a number that is half the sum of all of its positive divisors, including itself! This definition is at least 2300 years old as it appears in Euclid's "Elements".