Perfect, Amicable and Sociable Numbers  
   Definitions, links and references of various applications of the sum of divisors function s(n).
   http://xraysgi.ims.uconn.edu:8080/amicable.html

The Perfect Number Journey  
   A Mathematics Enrichment Workshop introducing perfect numbers. Lessons and exercises extend over several pages. Aimed at interested middle and high school students.
   http://home1.pacific.net.sg/%7Enovelway/MEW2/lesson1.html

Perfect Numbers  
   A detailed history of the quest for perfect numbers, from Euclid to their present-day Mersenne discoveries.
   http://www-history.mcs.st-and.ac.uk/history/HistTopics/Perfect_numbers.html

Cubes of Perfection  
   From Ivars Peterson's MathTrek column in MAA Online. Curious relationships satisfied by perfect numbers.
   http://www.maa.org/mathland/mathtrek_5_18_98.html

On a Generalization of Perfect Numbers  
   J. L. Pe introduces perfect numbers relative to an arithmetical function f. Under this scheme, the usual perfect numbers are just one among many species of "f-perfect numbers". Several open problems and examples of perfect number sets are given,
   http://www.geocities.com/windmill96/fperfect/fperfect.html

Amicable Numbers  
   Observations on amicable pairs and their distribution, including Harshad and Happy Amicable pairs.
   http://www.shyamsundergupta.com/amicable.htm

Multiply Perfect Numbers  
   Up-to-date information on multiperfect numbers and searchable database maintained by Achim Flammenkamp.
   http://wwwhomes.uni-bielefeld.de/achim/mpn.html

Tables of Aliquot Cycles  
   Data for various kinds up to 10^12, including unitary, infinitary, exponential, augmented and reduced varieties.
   http://amicable.homepage.dk/tables.htm

Aliquot  
   Sequences with initial term up to 10,000, by Christophe Clavier.
   http://christophe.clavier.free.fr/Aliquot/site/Aliquot.html

Perfect Number - from MathWorld  
   Definitions and references.
   http://mathworld.wolfram.com/PerfectNumber.html

Odd Perfect Numbers - Missing Values  
   Status of various factorisations that would extend the known area of non-existence of odd perfect numbers, by Kevin Hare.
   http://www.math.uwaterloo.ca/~kghare/ODDPERFECT/MissingValues.html

Aliquot Sequences  
   Aliquot sequences arise in iterating the sum-of-divisors function s. Bosma has calculated tables up to start value 50,000 and 80 digits.
   http://www.math.ru.nl/~bosma/research/nuth/ali.html