Aliquot Sequences
- Statistics by Wolfgang Creyaufmueller, including many tables and links to factoring sites and software.
- http://www.aliquot.de/aliquote.htm

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

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

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

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

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

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