A163800 a(n) is the n-th J_20-prime (Josephus_20 prime).

2, 5, 30, 54, 81, 109, 149, 186, 513, 1089, 8158, 8533, 17178, 34478, 913274, 976402

Offset: 1

Author

Peter R. J. Asveld, Aug 04 2009, Aug 12 2009

Keywords

• nonn
• more

Comments

Place the numbers 1..N (N>=2) on a circle and cyclicly mark the 20th unmarked number until all N numbers are marked. The order in which the N numbers are marked defines a permutation; N is a J_20-prime if this permutation consists of a single cycle of length N.

There are 16 J_20-primes in the interval 2..1000000 only. No formula is known; the J_20-primes were found by exhaustive search.

Examples

			2 is a J_20-prime (trivial).
		

References

• R. L. Graham, D. E. Knuth & O. Patashnik, Concrete Mathematics (1989), Addison-Wesley, Reading, MA. Sections 1.3 & 3.3.

Links

• Jean-Paul Allouche, Manon Stipulanti, and Jia-Yan Yao, Doubling modulo odd integers, generalizations, and unexpected occurrences, arXiv:2504.17564 [math.NT], 2025.
• P. R. J. Asveld, Permuting operations on strings and their relation to prime numbers, Discrete Applied Mathematics 159 (2011) 1915-1932.
• P. R. J. Asveld, Permuting Operations on Strings-Their Permutations and Their Primes, Twente University of Technology, 2014. University link.
• Index entries for sequences related to the Josephus Problem

Crossrefs

See A163782 through A163799 for J_2- through J_19-primes.