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A007615 Primes with unique period length (the periods are given in A007498).

%I A007615 M2890 #42 Jan 03 2024 23:46:24
%S A007615 3,11,37,101,333667,9091,9901,909091,1111111111111111111,
%T A007615 11111111111111111111111,99990001,999999000001,909090909090909091,
%U A007615 900900900900990990990991,9999999900000001,909090909090909090909090909091,900900900900900900900900900900990990990990990990990990990991
%N A007615 Primes with unique period length (the periods are given in A007498).
%C A007615 Additional terms are Phi(n,10)/gcd(n,Phi(n,10)) for the n in A007498, where Phi(n,10) is the n-th cyclotomic polynomial evaluated at 10.
%D A007615 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D A007615 Samuel Yates, Period Lengths of Exactly One or Two Prime Numbers, J. Rec. Math., 18 (1985), 22-24.
%H A007615 Max Alekseyev, <a href="/A007615/b007615.txt">Table of n, a(n) for n = 1..98</a> (terms 1..25 from T. D. Noe; terms 26..31 from Ray Chandler)
%H A007615 C. K. Caldwell, The Prime Glossary, <a href="https://t5k.org/glossary/page.php?sort=UniquePrime">unique prime</a>
%H A007615 Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit/phin10.htm">Factorizations of Phi_n(10)</a>
%H A007615 <a href="/index/1#1overn">Index entries for sequences related to decimal expansion of 1/n</a>
%F A007615 a(n) = A061075(A007498(n)). - _Max Alekseyev_, Oct 16 2010
%F A007615 a(n) = A006530(A019328(A007498(n))). - _Ray Chandler_, May 10 2017
%e A007615 3 is the only prime p such that decimal expansion of 1/p has (nontrivial) period exactly 1.
%t A007615 nmax = 50; periods = Reap[ Do[ p = Cyclotomic[n, 10] / GCD[n, Cyclotomic[n, 10]]; If[ PrimeQ[p], Sow[n]], {n, 1, nmax}]][[2, 1]]; Cyclotomic[#, 10] / GCD[#, Cyclotomic[#, 10]]& /@ periods // Prepend[#, 3]& (* _Jean-François Alcover_, Mar 28 2013 *)
%Y A007615 Cf. A007498, A040017, A002371, A048595, A006883, A007732, A051626, A061075, A006530, A019328.
%K A007615 nonn,nice,easy,base
%O A007615 1,1
%A A007615 _N. J. A. Sloane_, _Robert G. Wilson v_, _Mira Bernstein_