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A235863 Exponent of the multiplicative group G_n:={x+iy: x^2+y^2==1 (mod n); 0 <= x,y < n} where i=sqrt(-1). 7

%I

%S 1,2,4,4,4,4,8,4,12,4,12,4,12,8,4,4,16,12,20,4,8,12,24,4,20,12,36,8,

%T 28,4,32,8,12,16,8,12,36,20,12,4,40,8,44,12,12,24,48,4,56,20,16,12,52,

%U 36,12,8,20,28,60,4,60,32,24,16,12,12,68,16,24,8,72

%N Exponent of the multiplicative group G_n:={x+iy: x^2+y^2==1 (mod n); 0 <= x,y < n} where i=sqrt(-1).

%C From _Jianing Song_, Nov 05 2019: (Start)

%C Exponent of the group G is the least e > 0 such that x^e = 1 for every x in G, where 1 is the identity element.

%C Also the exponent of O(2,Z_n) or SO(2,Z_n). O(2,Z_n) is the group of 2 X 2 matrices A over Z_n such that A*A^T = E = [1,0;0,1]; SO(2,Z_n) is the group of 2 X 2 matrices A over Z_n such that A*A^T = E = [1,0;0,1] and det(A) = 1. Note that G_n is isomorphic to SO(2,Z_n) by the mapping x+yi <-> [x,y;-y,x]. See A060698 for the group structure of SO(2,Z_n) and A182039 for the group structure of O(2,Z_n). (End)

%H Andrew Howroyd, <a href="/A235863/b235863.txt">Table of n, a(n) for n = 1..10000</a>

%H José María Grau, A. M. Oller-Marcén, Manuel Rodriguez and Daniel Sadornil, <a href="http://arxiv.org/abs/1401.4708">Fermat test with Gaussian base and Gaussian pseudoprimes</a>, arXiv:1401.4708 [math.NT], 2014.

%H José María Grau, A. M. Oller-Marcén, Manuel Rodriguez and Daniel Sadornil, <a href="http://dx.doi.org/10.1007/s10587-015-0221-2">Fermat test with Gaussian base and Gaussian pseudoprimes</a>, Czechoslovak Mathematical Journal 65(140), (2015) pp. 969-982.

%F a(@) = 2, a(4) = a(8) = a(16) = 4, a(2^e) = 2^(e-2) for e >= 5; a(p^e) = (p-1)*p^(e-1) if e p == 1 (mod 4) and (p+1)*p^(e-1) if e p == 1 (mod 4). - _Jianing Song_, Nov 05 2019

%F If gcd(n,m)=1 then a(nm) = lcm(a(n), a(m)).

%t fa=FactorInteger; lam[1]=1;lam[p_, s_] := Which[Mod[p, 4] == 3, p ^ (s - 1 ) (p + 1) , Mod[p, 4] == 1, p ^ (s - 1 ) (p - 1) , s ≥ 5, 2 ^ (s - 2 ), s > 1, 4, s == 1, 2];lam[n_] := {aux = 1; Do[aux = LCM[aux, lam[fa[n][[i, 1]], fa[n][[i, 2]]]], {i, 1, Length[fa[n]]}]; aux}[[1]] ; Array[lam, 100]

%o (PARI) a(n)={my(f=factor(n)); lcm(vector(#f~, i, my([p,e]=f[i,]); if(p==2, 2^max(e-2, min(e,2)), p^(e-1)*if(p%4==1, p-1, p+1))))} \\ _Andrew Howroyd_, Aug 06 2018

%Y (Z/nZ)* ------ G_n

%Y Order: A000010 ------ A060968.

%Y Exponent: A002322 ------ this sequence.

%Y n-1 ------ A201629.

%Y Carmichael/G-Carmichael numbers: A002997 ------ A235865.

%Y Lehmer /G-Lehmer numbers: unknown ------ A235864.

%Y Cyclic/G-cyclic numbers: A003277 ------ A235866.

%Y n such that the group is cyclic: A033948 ------ A235868.

%Y Cf. A060698, A182039.

%K nonn

%O 1,2

%A _José María Grau Ribas_, Jan 16 2014

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Last modified May 28 01:34 EDT 2020. Contains 334671 sequences. (Running on oeis4.)