cp's OEIS Frontend

A308795 Primes p such that A001177(p) = (p-1)/2.

%I A308795 #13 Jul 05 2019 16:01:40
%S A308795 29,41,101,181,229,241,349,409,449,509,569,601,641,929,941,1021,1061,
%T A308795 1109,1129,1201,1229,1321,1481,1489,1549,1609,1621,1669,1709,1741,
%U A308795 1789,1801,1861,1889,2029,2069,2129,2609,2621,2861,3209,3301,3361,3389,3449,3461,3581
%N A308795 Primes p such that A001177(p) = (p-1)/2.
%C A308795 Primes p such that ord(-(3+sqrt(5))/2,p) = (p-1)/2, where ord(z,p) is the smallest integer k > 0 such that (z^k-1)/p is an algebraic integer.
%C A308795 Let {T(n)} be a sequence defined by T(0) = 0, T(1) = 1, T(n) = k*T(n-1) + T(n-2), K be the quadratic field Q[sqrt(k^2+4)], O_K be the ring of integer of K, u = (k+sqrt(k^2+4))/2. For a prime p not dividing k^2 + 4, the Pisano period of {T(n)} modulo p (that is, the smallest m > 0 such that T(n+m) == T(n) (mod p) for all n) is ord(u,p); the entry point of {T(n)} modulo p (that is, the smallest m > 0 such that T(m) == 0 (mod p)) is ord(-u^2,p).
%C A308795 For an odd prime p:
%C A308795 (a) if p decomposes in K, then (O_K/pO_K)* (the multiplicative group of O_K modulo p) is congruent to C_(p-1) X C_(p-1), so the entry point of {T(n)} modulo p is equal to (p-1)/s, s = 1, 2, 3, 4, ...;
%C A308795 (b) if p is inert in K, then u^(p+1) == -1 (mod p), (-u^2)^(p+1) == 1 (mod p), so the entry point of {T(n)} modulo p is equal to (p+1)/s, s = 1, 2, 3, 4, ...
%C A308795 Here k = 1, and this sequence gives primes such that (a) holds and s = 2. For even s, all terms are congruent to 1 modulo 4.
%C A308795 Number of terms below 10^N:
%C A308795   N | Number | Decomposing primes*
%C A308795   3 |     15 |            78
%C A308795   4 |    115 |           609
%C A308795   5 |    839 |          4777
%C A308795   6 |   6913 |         39210
%C A308795   7 |  58891 |        332136
%C A308795   8 | 510784 |       2880484
%C A308795   * Here "Decomposing primes" means primes such that Legendre(5,p) = 1, i.e., p == 1, 4 (mod 5).
%t A308795 pn[n_] := For[k = 1, True, k++, If[Mod[Fibonacci[k], n] == 0, Return[k]]];
%t A308795 Reap[For[p = 2, p < 3600, p = NextPrime[p], If[pn[p] == (p - 1)/2, Print[p]; Sow[p]]]][[2, 1]] (* _Jean-François Alcover_, Jul 05 2019 *)
%o A308795 (PARI) Entry_for_decomposing_prime(p) = my(k=1, M=[k, 1; 1, 0]); if(isprime(p)&&kronecker(k^2+4,p)==1, my(v=divisors(p-1)); for(d=1, #v, if((Mod(M,p)^v[d])[2,1]==0, return(v[d]))))
%o A308795 forprime(p=2, 3600, if(Entry_for_decomposing_prime(p)==(p-1)/2, print1(p, ", ")))
%Y A308795 Similar sequences that give primes such that (a) holds: A106535 (s=1), this sequence (s=2), A308796 (s=3), A308797 (s=4), A308798 (s=5), A308799 (s=6), A308800 (s=7), A308801 (s=8), A308802 (s=9).
%K A308795 nonn
%O A308795 1,1
%A A308795 _Jianing Song_, Jun 25 2019