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A300915
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Order of the group PSL(2,Z_n).
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1
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1, 6, 12, 24, 60, 72, 168, 96, 324, 360, 660, 288, 1092, 1008, 720, 768, 2448, 1944, 3420, 1440, 2016, 3960, 6072, 1152, 7500, 6552, 8748, 4032, 12180, 4320, 14880, 6144, 7920, 14688, 10080, 7776, 25308, 20520, 13104, 5760
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OFFSET
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1,2
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COMMENTS
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The projective special linear group PSL(2,Z_n) is the quotient group of SL(2,Z_n) with its center. The center of SL(2,Z_n) is the group of scalar matrices whose diagonal entry is x in Z_n such that x^2 = 1. The elements of PSL(2,Z_n) are equivalence classes of 2 X 2 matrices with entries in Z_n where two matrices are equivalent if one is a scalar multiple of the other.
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LINKS
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FORMULA
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Multiplicative with a(2) = 6, a(2^2) = 24, a(2^e) = 3*2^(3*e-4) for e > 2, and a(p^e) = (p^2-1)*p^(3*e-2)/2 for p > 2. - Amiram Eldar, Dec 01 2022
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MATHEMATICA
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n := 2; nn = 40; \[Gamma][n_, q_] := Product[q^n - q^i, {i, 0, n - 1}]; Prepend[ Table[Product[ FactorInteger[m][[All, 1]][[j]]^(n^2 (FactorInteger[m][[All, 2]][[j]] - 1)) \[Gamma][n, FactorInteger[m][[All, 1]][[j]]], {j, 1, PrimeNu[m]}], {m, 2, nn}]/Table[EulerPhi[m], {m, 2, nn}]/ Table[Count[Mod[Select[Range[m], GCD[#, m] == 1 &]^n, m], 1], {m, 2, nn}], 1]
f[p_, e_] := (p^2-1)*p^(3*e-2)/2; f[2, e_] := Switch[e, 1, 6, 2, 24, _, 3*2^(3*e-4)]; a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 40] (* Amiram Eldar, Dec 01 2022 *)
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PROG
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(PARI) a(n) = {my(f=factor(n)); prod(i=1, #f~, my([p, e]=f[i, ]); (p^2-1)*p^(3*e-2)/if(p==2, 2^min(2, e-1), 2))} \\ Andrew Howroyd, Aug 01 2018
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CROSSREFS
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KEYWORD
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nonn,mult
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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