A positive integer is a perfect number if it equals the sum of all its divid
ers. For example
6 = 1 + 2 + 3
28 = 1+2+4+7+14
A conjecture is to prove that there is infinite number of perfect numbers. I
t remained open till late 90s. I do not know if there is progress on this re
cently.
I read this in a Chinese book during high school.
