An Image from the Collatz Problem
- By Andrew Shapira. The intensity of a point denotes the time taken to terminate.
- http://www.onezero.org/collatz-image

The 3x + 1 Problem and its Generalizations
- An activated-text survey article by Jeff Lagarias.
- http://oldweb.cecm.sfu.ca/organics/papers/lagarias/

The Collatz 3n+1 Conjecture is Unprovable
- A paper by Craig Alan Feinstein arguing that the Collatz Conjecture cannot be formally proved.
- http://arxiv.org/abs/math.GM/0312309

The 3x+1 Problem Annotated Bibliography
- By Jeffrey Lagarias, 1997.
- http://www.math.lsa.umich.edu/~lagarias/3x+1.html

Experiments with the 3n+1 Sequence
- An online calculator by Alfred Wassermann.
- http://did.mat.uni-bayreuth.de/personen/wassermann/fun/3np1_e.html

The Generalised 3x+1 Problem
- A survey by Keith Matthews [PDF].
- http://www.numbertheory.org/pdfs/survey.pdf

Computational Verification of the 3x+1 Conjecture
- Results up to 2^60 by Tomás Oliveira e Silva.
- http://www.ieeta.pt/~tos/3x+1.html

The 3x+1 Problem Conference Proceedings
- Katholische Universität Eichstätt, Germany; 5--6 August 1999. On-line proceedings and group photo.
- http://www.math.grin.edu/~chamberl/conf.html

Collatz Problem
- From Eric Weissten's World of Mathematics. Article with references and links.
- http://mathworld.wolfram.com/CollatzProblem.html

Collatz 3n+1 Problem Structure
- Observations posted by Ken Conrow to stimulate further research.
- http://www-personal.ksu.edu/~kconrow/

The Structure of the 3x + 1 Function
- Papers by Peter Schorer describing several new approaches.
- http://www.occampress.com/