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a(n) is the n-th J_20-prime (Josephus_20 prime).
19

%I #24 Aug 05 2024 14:07:22

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

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

%C 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.

%C 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.

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

%H P. R. J. Asveld, <a href="http://dx.doi.org/10.1016/j.dam.2011.07.019">Permuting operations on strings and their relation to prime numbers</a>, Discrete Applied Mathematics 159 (2011) 1915-1932.

%H P. R. J. Asveld, <a href="https://citeseerx.ist.psu.edu/pdf/9d8542763057ef03a22b57f87085d69497ddaf46">Permuting Operations on Strings-Their Permutations and Their Primes</a>, Twente University of Technology, 2014. <a href="http://doc.utwente.nl/67513">University link</a>.

%H <a href="/index/J#Josephus">Index entries for sequences related to the Josephus Problem</a>

%e 2 is a J_20-prime (trivial).

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

%K nonn,more

%O 1,1

%A _Peter R. J. Asveld_, Aug 04 2009, Aug 12 2009