Blecksmith, Richard and Erdős, Paul and Selfridge, J. L., Cluster primes. Amer. Math. Monthly (1999), 43-48.
El03
Elsholtz, Christian, On cluster primes. Acta Arith. (2003), 281-284.
Gu04
Guy, Richard K., Unsolved problems in number theory. Third edition, Problem Books in Mathematics, Springer (2004), xviii+437 pp. Section A8 "Gaps between primes. Twin primes.", printed p. 36: the cluster primes, 97 the smallest non-cluster prime, and Blecksmith and Selfridge's question whether there are infinitely many; the same question is the second definition of a good prime in A14 "“Good” primes and the prime number graph", printed p. 54. Library home: Guy 2004.