A number is perfect (or deficient or abundant) as it is equal to (or less or more) than the sum of its aliquot parts (its factors). For example,
6 is perfect as 6=1+2+3;
8 is deficient as 8>1+2+4 or
12 is abundant as 12=1+2+3+4+6.
Euclid proved (in Elements IX, 36) that if and is prime, then is perfect. Some of the discovered perfect numbers are the following:
Fibonacci (1202) draw a rule whereby
where is prime
a rule which holds for all known perfect numbers.
The fifth perfect number is given as 33 550 336 in an anonymous manuscript in 1456-1461.
The Pythagoreans used this term in another sense, because apparently 10 was considered by them to be a perfect number.
Proposition 36 of Book IX of Euclid's Elements is:
Perfect number appears in English in 1570 in Sir Henry Billingsley's translation of Euclid.
What people thought of primes through the history
Prime number (as the one defined by Aristotle, Euclid and Theon of Smyrna) is a number "measured by no number but by an unit alone" Iambilicus said that a prime number is also called "odd times odd".
Prime number was apparently first described by Pythagoras.
Iamblichus writes that Thymaridas called a prime number rectilinear since it can only be represented one-dimensionally.
In English prime number is found in Sir Henry Billingsley's 1570 translation of Euclid's Elements (OED2).
Some older textbooks include 1 as a prime number.
In his Algebra (1770), Euler did not consider 1 a prime [William C. Waterhouse].
Learn something more about perfect numbers by looking at abundant and deficient numbers page.