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: Paul Erdős Erdős published at least 1,525 mathematical papers during his lifetime. He engaged more than 500 collaborators and devoted most of his time to mathematics up until his death in 1996 at the age of 83 years old.
Answer: 3 Of course, as time passes, the smallest Erdős number that can still be achieved will necessarily increase, as mathematicians with low Erdős numbers die and become unavailable for collaboration.
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".
