A335005 -id:A335005 - cp's OEIS frontend

A235384 Number of involutions in the group Aff(Z/nZ).

2, 4, 6, 6, 8, 8, 16, 10, 12, 12, 24, 14, 16, 24, 28, 18, 20, 20, 36, 32, 24, 24, 64, 26, 28, 28, 48, 30, 48, 32, 52, 48, 36, 48, 60, 38, 40, 56, 96, 42, 64, 44, 72, 60, 48, 48, 112, 50, 52, 72, 84, 54, 56, 72, 128, 80, 60, 60, 144, 62, 64, 80, 100, 84

Offset: 2

Tom Edgar, Jan 08 2014

Aff(Z/nZ) is the group of functions on Z/nZ of the form x->ax+b where a and b are elements of Z/nZ and gcd(a,n)=1.
Since Aff(Z/nZ) is isomorphic to the automorphism group of the dihedral group with 2n elements (when n>=3), this is the number of automorphisms of the dihedral group with 2n elements that have order 1 or 2.
The sequence is multiplicative: a(k*m) = a(k)*a(m) if m and k are coprime.
When n=26, this is the number of affine ciphers where encryption and decryption use the same function.

			Since 18 = 2*3^2, we get a(18) = 2*(3^2+1) = 20. Since 120 = 2^3*3*5, we have a(120) = (4+2^2+2^3)*(3+1)*(5+1) = 384.

Alois P. Heinz, Table of n, a(n) for n = 2..10000
K. K. A. Cunningham, Tom Edgar, A. G. Helminck, B. F. Jones, H. Oh, R. Schwell and J. F. Vasquez, On the Structure of Involutions and Symmetric Spaces of Dihedral Groups, Note di Mat., Volume 34, No. 2, 2014.

Cf. A002618, A228179, A147848, A060594, A283796, A335005.

a:= n-> add(`if`(irem(k^2, n)=1, igcd(n, k+1), 0), k=1..n-1):
seq(a(n), n=2..100);  # Alois P. Heinz, Jan 20 2014

a[n_] := Sum[If[Mod[k^2, n] == 1, GCD[n, k+1], 0], {k, 1, n-1}]; Table[a[n], {n, 2, 100}] (* Jean-François Alcover, Mar 24 2014, after Alois P. Heinz *)
f[p_, e_] := p^e + 1; f[2, 1] = 2; f[2, 2] = 6; f[2, e_] := 3*2^(e - 1) + 4; a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100, 2]  (* Amiram Eldar, Dec 05 2022 *)

A034448(n,f=factor(n))=factorback(vector(#f~,i,f[i,1]^f[i,2]+1))
a(n)=my(m=valuation(n,2)); if(m==0,1,m==1,2,m==2,6,4+3<<(m-1))*A034448(n>>m) \\ Charles R Greathouse IV, Jul 29 2016

def a(n):
    L=list(factor(n))
    if L[0][0]==2:
        m=L[0][1]
        L.pop(0)
    else:
        m=0
    order=prod([x[0]^x[1]+1 for x in L])
    if m==1:
        order=2*order
    elif m==2:
        order=6*order
    elif m>=3:
        order=(4+2^(m-1)+2^m)*order
    return order
[a(i) for i in [2..100]]

def b(n):
    sum = 0
    for a in [x for x in range(n) if ((x^2) % n == 1)]:
        sum += gcd(a+1,n)
    return sum
[b(i) for i in [2..100]]

Suppose n = 2^m*p_1^(r_1)*p_2^(r_2)*...*p_k^(r_k) where each p_i>2 is prime, r_i>0 for all i, and m>=0 is the prime factorization of n, then:
...a(n) = Product_{1<=i<=k} (p_i^(r_i)+1) if m=0,
...a(n) = 2*Product_{1<=i<=k} (p_i^(r_i)+1) if m=1,
...a(n) = 6*Product_{1<=i<=k} (p_i^(r_i)+1) if m=2,
...a(n) = (4+2^(m-1)+2^m)*Product_{1<=i<=k} (p_i^(r_i)+1) if m>=3.
a(n) = Sum_{a in row(n) of A228179} gcd(a+1,n).
Sum_{k=1..n} a(k) ~ c * n^2, where c = zeta(2)/(2*zeta(3)) = 0.684216... (A335005). - Amiram Eldar, Dec 05 2022

A343442 If n = Product (p_j^k_j) then a(n) = Product (p_j + 2), with a(1) = 1.

Original entry on oeis.org

1, 4, 5, 4, 7, 20, 9, 4, 5, 28, 13, 20, 15, 36, 35, 4, 19, 20, 21, 28, 45, 52, 25, 20, 7, 60, 5, 36, 31, 140, 33, 4, 65, 76, 63, 20, 39, 84, 75, 28, 43, 180, 45, 52, 35, 100, 49, 20, 9, 28, 95, 60, 55, 20, 91, 36, 105, 124, 61, 140, 63, 132, 45, 4, 105, 260, 69, 76, 125, 252

Offset: 1

Ilya Gutkovskiy, Apr 15 2021

Cf. A000203, A001615, A007429, A007947, A008683, A048250, A062953, A063441, A074816, A092261, A107758, A107759, A319132, A335005.

a[1] = 1; a[n_] := Times @@ ((#[[1]] + 2) & /@ FactorInteger[n]); Table[a[n], {n, 70}]
nmax = 70; CoefficientList[Series[Sum[MoebiusMu[k]^2 DivisorSigma[1, k] x^k/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest

a(n) = sumdiv(n, d, moebius(d)^2 * sigma(d)) \\ Andrew Howroyd, Apr 15 2021

G.f.: Sum_{k>=1} mu(k)^2 * sigma(k) * x^k / (1 - x^k), where mu = A008683 and sigma = A000203.
a(n) = Sum_{d|n} mu(d)^2 * sigma(d).
Sum_{k=1..n} a(k) ~ c * n^2, where c = Pi^2/(12*zeta(3)) = 0.684216... (A335005). - Amiram Eldar, Nov 13 2022
a(n) = Sum_{d|n} mu(d)^2*psi(d), where psi is A001615. - Ridouane Oudra, Jul 24 2025

cp's OEIS Frontend

A235384 Number of involutions in the group Aff(Z/nZ).

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