Gardner, Martin (1978), "The Bells: versatile numbers that can count partitions of a set, primes and even rhymes", Scientific American, 238: 24–30, doi:10.1038/scientificamerican0578-24. Reprinted with an addendum as "The Tinkly Temple Bells", Chapter 2 of Fractal Music, Hypercards, and more ... Mathematical Recreations from Scientific American, W. H. Freeman, 1992, pp. 24–38.
