

A306074


Bases in which 5 is a uniqueperiod prime.


5



2, 3, 4, 6, 7, 9, 19, 24, 26, 39, 49, 79, 99, 124, 126, 159, 199, 249, 319, 399, 499, 624, 626, 639, 799, 999, 1249, 1279, 1599, 1999, 2499, 2559, 3124, 3126, 3199, 3999, 4999, 5119, 6249, 6399, 7999, 9999, 10239, 12499, 12799, 15624, 15626, 15999, 19999, 20479
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OFFSET

1,1


COMMENTS

A prime p is called a uniqueperiod prime in base b if there is no other prime q such that the period length of the baseb expansion of its reciprocal, 1/p, is equal to the period length of the reciprocal of q, 1/q.
A prime p is a uniqueperiod prime in base b if and only if Zs(b, 1, ord(b,p)) = p^k, k >= 1. Here Zs(b, 1, d) is the greatest divisor of b^d  1 that is coprime to b^m  1 for all positive integers m < d, and ord(b,p) is the multiplicative order of b modulo p.
b is a term if and only if: (a) b = 5^t + 1, t >= 1; (b) b = 2^s*5^t  1, s >= 0, t >= 1; (c) b = 2, 3, 7.
For every odd prime p, p is a a uniqueperiod prime in base b if b = p^t + 1, t >= 1 or b = 2^s*p^t  1, s >= 0, t >= 1. These are trivial bases in which p is a uniqueperiod prime, with ord(b,p) = 1 or 2. By Faltings's theorem, there are only finitely many nontrivial bases in which p is also a uniqueperiod prime, with ord(b,p) >= 3. For p = 5, the nontrivial bases are 2, 3, 7.


LINKS

Jianing Song, Table of n, a(n) for n = 1..544
Wikipedia, Unique prime


EXAMPLE

1/5 has period length 4 in base 2. Note that 3 and 5 are the only prime factors of 2^4  1 = 15, but 1/3 has period length 2, so 5 is a uniqueperiod prime in base 2.
1/5 has period length 4 in base 3. Note that 2 and 5 are the only prime factors of 3^4  1 = 80, but 1/2 has period length 1, so 5 is a uniqueperiod prime in base 3.
1/5 has period length 4 in base 7. Note that 2, 3 and 5 are the only prime factors of 7^4  1 = 2400, but 1/2 and 1/3 both have period length 1, so 5 is a uniqueperiod prime in base 7.


PROG

(PARI)
p = 5;
gpf(n)=if(n>1, vecmax(factor(n)[, 1]), 1);
test(n, q)=while(n%p==0, n/=p); if(q>1, while(n%q==0, n/=q)); n==1;
for(n=2, 10^6, if(gcd(n, p)==1, if(test(polcyclo(znorder(Mod(n, p)), n), gpf(znorder(Mod(n, p)))), print1(n, ", "))));


CROSSREFS

Cf. A040017 (uniqueperiod primes in base 10), A144755 (base 2).
Bases in which p is a uniqueperiod prime: A000051 (p=2), A306073 (p=3), this sequence (p=5), A306075 (p=7), A306076 (p=11), A306077 (p=13).
Sequence in context: A224482 A002475 A208281 * A250252 A246274 A057519
Adjacent sequences: A306071 A306072 A306073 * A306075 A306076 A306077


KEYWORD

easy,nonn


AUTHOR

Jianing Song, Jun 19 2018


STATUS

approved



