Here's a classic paper on spin networks that was previously available only in an out-of-print book:
This electronic version was prepared by Georg Beyerle. Roger Penrose has given me his permission to make it available here.
Here's another, also prepared by Georg Beyerle:
Here are some notes, apparently passed from Roger Penrose to Lee Smolin, then to Scott Morrison, and then to me:
Related papers include:
If you find typos in either of the papers I'm making available, please let me know! Various people have caught typos so far; for example, Milan Vandrovec caught a lot of them. Note: above Figure 2 of "Angular momentum: an approach to combinatorial space-time" the list of numbers "3, 2, 3" is in the original paper. This might be a typo for "1, 2, 3", although "0, 1, 2, 3" would make even more sense.
- Roger Penrose, Applications of negative dimensional tensors, in Combinatorial Mathematics and its Applications, ed. D. Welsh, Academic Press, New York, 1971, pp. 221–244. Available on Louis Kauffman's website.
- R. Penrose, Combinatorial quantum theory and quantized directions, in Advances in Twistor Theory, eds. L. Hughston and R. Ward, Pitman Advanced Publishing Program, San Francisco, 1979, pp. 301–317.