|
|
A261580
|
|
Primes such that z(p) is odd where z(n) is A214028(n).
|
|
13
|
|
|
5, 13, 29, 37, 53, 61, 101, 109, 137, 149, 157, 173, 181, 197, 229, 269, 277, 293, 317, 349, 373, 389, 397, 421, 461, 509, 521, 541, 557, 569, 593, 613, 653, 661, 677, 701, 709, 733, 757, 773, 797, 821, 829, 853, 857, 877, 941, 953, 997, 1013, 1021, 1061, 1069
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
Primes p with 4 zeros in a fundamental period of A000129 mod p, that is, primes p such that A214027(p) = 4.
For p > 2, p is in this sequence if and only if (all these conditions are equivalent):
(b) ord(p,1+sqrt(2)) == 4 (mod 8), where ord(p,u) is the smallest integer k > 0 such that (u^k - 1)/p is an algebraic integer;
(c) ord(p,3+2*sqrt(2)) == 2 (mod 4);
(e) ord(p,-(3+2*sqrt(2))) is odd.
In general, let {x(n)} be a sequence defined by x(0) = 0, x(1) = 1, x(n+2) = m*x(n+1) + x(n). Let pi(k) be the Pisano period of {x(n)} modulo k, i.e., pi(k) = min{l > 0 : x(n+l) == x(n) (mod k) for all n}, r(k) = min{l > 0 : k divides x(l)} and w(k) be the number of zeros in a fundamental period of {x(n)} modulo k. Let u = (m + sqrt(m^2+4))/2, p be an odd prime, then these conditions are equivalent:
(1) w(p) = 4;
(2) pi(p) == 4 (mod 8);
(3) ord(p,u) == 4 (mod 8);
(4) ord(p,u^2) == 2 (mod 4);
(5) r(p) is odd;
(6) ord(p,-u^2) is odd.
This can be shown by noting that pi(p) = p^c*ord(p,u) and r(p) = p^c*ord(p,-u^2) for some c (if p does not divide m^2 + 4 then c = 0, otherwise c = 1). Also, Pi(p) is always even, so ord(p,u) is always even.
This sequence contains all primes congruent to 5 modulo 8.
Conjecturely, this sequence has density 7/24 in the primes. (End)
|
|
LINKS
|
|
|
EXAMPLE
|
The smallest Pell number divisible by the prime 5 has index 3, which is odd, so 5 is in the sequence.
|
|
MATHEMATICA
|
f[n_] := Block[{k = 1}, While[Mod[Simplify[((1 + Sqrt@ 2)^k - (1 - Sqrt@ 2)^k)/(2 Sqrt@ 2)], n] != 0, k++]; k]; Select[Prime@ Range@ 180, OddQ@ f@ # &] (* Michael De Vlieger, Aug 25 2015 *)
|
|
PROG
|
(PARI) pell(n) = polcoeff(Vec(x/(1-2*x-x^2) + O(x^(n+1))), n);
z(n) = {k=1; while (pell(k) % n, k++); k; }
lista(nn) = {forprime(p=2, nn, if (z(p) % 2, print1(p, ", ")); ); }
|
|
CROSSREFS
|
Let {x(n)} be the sequence defined in the comment section.
| m=1 | m=2 | m=3
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|