cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-7 of 7 results.

A002618 a(n) = n*phi(n).

Original entry on oeis.org

1, 2, 6, 8, 20, 12, 42, 32, 54, 40, 110, 48, 156, 84, 120, 128, 272, 108, 342, 160, 252, 220, 506, 192, 500, 312, 486, 336, 812, 240, 930, 512, 660, 544, 840, 432, 1332, 684, 936, 640, 1640, 504, 1806, 880, 1080, 1012, 2162, 768, 2058, 1000
Offset: 1

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Author

Keywords

Comments

Also Euler phi function of n^2.
For n >= 3, a(n) is also the size of the automorphism group of the dihedral group of order 2n. This automorphism group is isomorphic to the group of transformations x -> ax + b, where a, b and x are integers modulo n and a is coprime to n. Its order is n*phi(n). - Ola Veshta (olaveshta(AT)my-deja.com), Mar 18 2001
Order of metacyclic group of polynomial of degree n. - Artur Jasinski, Jan 22 2008
It appears that this sequence gives the number of permutations of 1, 2, 3, ..., n that are arithmetic progressions modulo n. - John W. Layman, Aug 27 2008
The conjecture by Layman is correct. Obviously any such permutation must have an increment that is prime to n, and almost as obvious that any such increment will work, with any starting value; hence phi(n) * n total. - Franklin T. Adams-Watters, Jun 09 2009
Consider the numbers from 1 to n^2 written line by line as an n X n square: a(n) = number of numbers that are coprime to all their horizontal and vertical immediate neighbors. - Reinhard Zumkeller, Apr 12 2011
n -> a(n) is injective: a(m) = a(n) implies m = n. - Franz Vrabec, Dec 12 2012 (See Mathematics Stack Exchange link for a proof.)
a(p) = p*(p-1) a pronic number, see A036689 and A002378. - Fred Daniel Kline, Mar 30 2015
Conjecture: All the rational numbers Sum_{i=j..k} 1/a(i) with 0 < min{2,k} <= j <= k have pairwise distinct fractional parts. - Zhi-Wei Sun, Sep 24 2015
From Jianing Song, Aug 25 2023: (Start)
a(n) is the order of the holomorph (see the Wikipedia link) of the cyclic group of order n. Note that Hol(C_n) and Aut(D_{2n}) are isomorphic unless n = 2, where D_{2n} is the dihedral group of order 2*n. See the Wordpress link.
The odd-indexed terms form a subsequence of A341298: the holomorph of an abelian group of odd order is a complete group. See Theorem 3.2, Page 618 of the W. Peremans link. (End)

Examples

			a(4) = 8 since phi(4) = 2 and 4 * 2 = 8.
a(5) = 20 since phi(5) = 4 and 5 * 4 = 20.
		

References

  • Peter Giblin, Primes and Programming: An Introduction to Number Theory with Computing. Cambridge: Cambridge University Press (1993) p. 116, Exercise 1.10.
  • J. L. Lagrange, Oeuvres, Vol. III Paris 1869.
  • N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
  • N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Crossrefs

First column of A047916.
Cf. A002619, A011755 (partial sums), A047918, A000010, A053650, A053191, A053192, A036689, A058161, A009262, A082473 (same terms, sorted into ascending order), A256545, A327172 (a left inverse), A327173, A065484.
Subsequence of A323333.

Programs

Formula

Multiplicative with a(p^e) = (p-1)*p^(2e-1). - David W. Wilson, Aug 01 2001
Dirichlet g.f.: zeta(s-2)/zeta(s-1). - R. J. Mathar, Feb 09 2011
a(n) = A173557(n) * A102631(n). - R. J. Mathar, Mar 30 2011
From Wolfdieter Lang, May 12 2011: (Start)
a(n)/2 = A023896(n), n >= 2.
a(n)/2 = (1/n) * Sum_{k=1..n-1, gcd(k,n)=1} k, n >= 2 (see A023896 and A076512/A109395). (End)
a(n) = lcm(phi(n^2),n). - Enrique Pérez Herrero, May 11 2012
a(n) = phi(n^2). - Wesley Ivan Hurt, Jun 16 2013
a(n) = A009195(n) * A009262(n). - Michel Marcus, Oct 24 2013
G.f.: -x + 2*Sum_{k>=1} mu(k)*k*x^k/(1 - x^k)^3. - Ilya Gutkovskiy, Jan 03 2017
a(n) = A082473(A327173(n)), A327172(a(n)) = n. -- Antti Karttunen, Sep 29 2019
Sum_{n>=1} 1/a(n) = 2.203856... (A065484). - Amiram Eldar, Sep 30 2019
Define f(x) = #{n <= x: a(n) <= x}. Gabdullin & Iudelevich show that f(x) ~ c*sqrt(x) for c = Product_{p prime} (1 + 1/(p*(p - 1 + sqrt(p^2 - p)))) = 1.3651304521525857... - Charles R Greathouse IV, Mar 16 2022
a(n) = Sum_{d divides n} A001157(d)*A046692(n/d); that is, the Dirichlet convolution of sigma_2(n) and the Dirichlet inverse of sigma_1(n). - Peter Bala, Jan 26 2024

Extensions

Better description from Labos Elemer, Feb 18 2000

A051625 Number of "labeled" cyclic subgroups of symmetric group S_n.

Original entry on oeis.org

1, 2, 5, 17, 67, 362, 2039, 14170, 109694, 976412, 8921002, 101134244, 1104940280, 13914013024, 191754490412, 2824047042632, 41304021782824, 708492417746000, 11629404776897384, 222093818836736752, 4351196253952132832, 88481681599705382144, 1781763397966126421200
Offset: 1

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Author

Keywords

Comments

Number of unordered lists of powers of permutation of length n (equivalent to the definition). - Olivier Gérard, Jul 04 2011
Number of subgroups of S_n with different permutations generated by single permutation (see Mathematica procedure). - Artur Jasinski, Oct 27 2011

Examples

			The 5 cyclic subgroups of symmetric group S_3 are: {Id}, the 3 subgroups {Id,(a,b)}, {Id,(b,c)}, {Id,(a,c)} and the Alternating group A_3: <Id, (a,b,c), (a,c,b)>.
The 17 cyclic subgroups of symmetric group S_4 are: {Id}, the 6 subgroups of type <(a,b)>, the 3 subgroups of type <(a,b)(c,d)>, the 4 subgroups of type <(a,b,c)> and the 3 subgroups of type <(a,b,c,d)>. - _Bernard Schott_, Feb 25 2019
		

References

  • V. Jovovic, Some combinatorial characteristics of symmetric and alternating groups (in Russian), Belgrade, 1980, unpublished.

Crossrefs

Row sums of A074881.

Programs

  • Maple
    parts:= proc(n,k) option remember;
       if k = 1 then return {[n]} fi;
       `union`(seq(map(t -> [op(t),j], procname(n-j*k,k-1)), j=0..floor(n/k)))
    end proc:
    F:= n -> add(n!/mul(p[k]!*k^p[k],k=1..nops(p)) / numtheory:-phi(ilcm(op(select(t -> p[t]<>0, [$1..n])))), p = parts(n,n)):
    seq(F(n),n=1..30); # Robert Israel, Oct 04 2015
  • Mathematica
    cc = {}; Do[aa = {}; kk = Table[n, {n, 1, ord}]; pp = Permutations[kk]; Do[per17 = {}; AppendTo[per17, pp[[p]]]; run = 0; ile = Length[per17]; min = 1; max = ile; While[ile < ord!, run = run + 1; if = False; Do[Do[vec0 = Table[0, {n, 1, ord}]; Do[vec0[[per17[[k]][[n]]]] = per17[[m]][[n]], {n, 1, ord}]; bp = vec0; If[Position[per17, bp] == {}, ile = ile + 1; Print[ile]; if = True; AppendTo[per17, bp]]; vec0 = Table[0, {n, 1, ord}]; Do[vec0[[per17[[m]][[n]]]] = per17[[k]][[n]], {n, 1, ord}]; bl = vec0; If[Position[per17, bl] == {}, ile = ile + 1; if = True; AppendTo[per17, bl]]; If[ile == ord!, Break[]], {k, 1, max}], {m, min, max}]; If[if == False, Break[], min = max + 1; max = ile]]; AppendTo[aa, Sort[per17]], {p, 1, ord!}]; AppendTo[cc, Length[Union[aa]]], {ord, 1, 7}]; cc (* Artur Jasinski, Oct 27 2011 *)
    permcount[v_] := Module[{m=1, s=0, k=0, t}, For[i=1, i <= Length[v], i++, t = v[[i]]; k = If[i>1 && t == v[[i-1]], k+1, 1]; m *= t*k; s += t]; s!/m];
    a[n_] := Module[{s = 0}, Do[s += permcount[p]/EulerPhi[LCM @@ p], {p, IntegerPartitions[n]}]; s];
    Array[a, 23] (* Jean-François Alcover, Feb 25 2019, after Andrew Howroyd *);
    content[li_List] := Table[Count[li, i], {i, Tr[li]}]; Table[Tr[(n!/(Times @@ (Range[Tr[#1]]^content[#1]*content[#1]!)*EulerPhi[LCM @@ Flatten[Position[content[#1], ?Positive]]]) & ) /@ IntegerPartitions[n] ], {n, 23}] (* _Wouter Meeussen, Jan 06 2021 *);
  • PARI
    \\ permcount is number of permutations of given type.
    permcount(v) = {my(m=1,s=0,k=0,t); for(i=1,#v,t=v[i]; k=if(i>1&&t==v[i-1],k+1,1); m*=t*k;s+=t); s!/m}
    a(n)={my(s=0); forpart(p=n, s+=permcount(p)/eulerphi(lcm(Vec(p)))); s} \\ Andrew Howroyd, Jul 03 2018

Formula

a(n) = Sum_{pi} n!/(k_1!*1^k_1*k_2!*2^k_2*...*k_n!*n^k_n*phi(lcm{i:k_i != 0})), where pi runs through all partitions k_1+2*k_2+...+n*k_n=n and phi is Euler's function.

A058162 Number of labeled Abelian groups with a fixed identity.

Original entry on oeis.org

1, 1, 1, 4, 6, 60, 120, 1920, 7560, 90720, 362880, 13305600, 39916800, 1037836800, 10897286400, 265686220800, 1307674368000, 66691392768000, 355687428096000, 20274183401472000, 202741834014720000
Offset: 1

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Author

Christian G. Bower, Nov 15 2000, Mar 12 2008

Keywords

Comments

The distinction here between labeled and unlabeled Abelian groups is analogous to the distinction between unlabeled rooted trees (A000081) and labeled rooted trees (A000169).
That is, the number of Cayley tables. - Artur Jasinski, Mar 12 2008
Number of Latin squares in dimension n with first row and first column 1,2,3 ..., n which are associative and commutative (Abelian). Each of these squares is isomorphic with the Cayley table of one of the existed Abelian group in dimension n. - Artur Jasinski, Nov 02 2005. Cf. A111341.

Examples

			The 2 unlabeled Abelian groups of order 4 are C4 and C2^2. The 4 labeled Abelian groups whose identity is "0" consist of 3 of type C4 (where the nongenerator can be "2", "3", or "4") and 1 of type C2^2.
		

Crossrefs

Formula

a(n) = A034382(n) / n. Formula for A034382 is based on the fundamental theorem of finite Abelian groups and the formula given by Hillar and Rhea (2007).

Extensions

a(16) and a(21) corrected by Max Alekseyev, Sep 12 2019

A034381 Number of labeled cyclic groups with n elements.

Original entry on oeis.org

1, 2, 3, 12, 30, 360, 840, 10080, 60480, 907200, 3991680, 119750400, 518918400, 14529715200, 163459296000, 2615348736000, 22230464256000, 1067062284288000, 6758061133824000, 304112751022080000, 4257578514309120000, 112400072777760768000, 1175091669949317120000
Offset: 1

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Author

Keywords

Comments

This sequence is strictly increasing, since a(n) = n!/phi(n) > n!/n = (n-1)! >= a(n-1) for n >= 2. - Jianing Song, Mar 02 2024

Crossrefs

Programs

Formula

a(n) = n!/phi(n).
a(n) = A000142(n)/A000010(n) = n*A058161(n).

Extensions

a(21) onwards from Jianing Song, Mar 02 2024

A137149 a(n) = (prime(n)-2)!.

Original entry on oeis.org

1, 1, 6, 120, 362880, 39916800, 1307674368000, 355687428096000, 51090942171709440000, 10888869450418352160768000000, 8841761993739701954543616000000
Offset: 1

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Author

Artur Jasinski, Jan 23 2008

Keywords

Comments

Old definition was "a(n) = prime(n)!/(prime(n)*EulerPhi(prime(n)))".
Degree of Lagrange resolvent of polynomial prime degree. Ratio: degree of symmetric group of prime order n divided by order metacyclic group of prime order n. For degree of Lagrange resolvent of polynomial not prime degree see A137150.

Crossrefs

Programs

  • Magma
    [Factorial(NthPrime(n)-2): n in [1..15]]; // Vincenzo Librandi, May 04 2014
  • Mathematica
    Table[(Prime[n]-2)!, {n, 1, 15}] (* Bruno Berselli, May 04 2014 *)

Extensions

New name from Vincenzo Librandi, May 04 2014

A137150 Degree of Lagrange resolvent of polynomial of composite degree.

Original entry on oeis.org

1, 3, 60, 1260, 6720, 90720, 9979200, 1037836800, 10897286400, 163459296000, 59281238016000, 15205637551104000, 202741834014720000, 5109094217170944000, 3231502092360622080000, 31022420086661971968000
Offset: 1

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Author

Artur Jasinski, Jan 23 2008

Keywords

Comments

Ratio: degree of symmetric group of composite order n divided by order metacyclic group of composite order n.

Crossrefs

Programs

  • Mathematica
    a = {}; Do[If[PrimeQ[n],[null], AppendTo[a, n!/(n EulerPhi[n])]], {n, 1, 30}]; a
    With[{nn=30},#!/(# EulerPhi[#])&/@Complement[Range[nn],Prime[Range[ PrimePi[ nn]]]]] (* Harvey P. Dale, Jul 05 2014 *)

Formula

a(n) = n!/(n EulerPhi[n]) for composite n A058161 = A137149 + A137150.

A336412 Number of labeled dihedral groups with a fixed identity.

Original entry on oeis.org

1, 1, 20, 630, 18144, 3326400, 148262400, 40864824000, 6586804224000, 3041127510220800, 464463110651904000, 538583682060103680000, 99430833611096064000000, 129629398219266097152000000, 73681349947830849621196800000, 64240926985765022013480960000000
Offset: 1

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Author

Dan Eilers, Jul 20 2020

Keywords

Comments

a(n) is the number of dihedral groups of order 2n with a fixed identity, or equivalently the number of reduced Latin squares of order 2n that can be viewed as the Cayley table of D_{2n}, by adding a border that matches the first row and column. The reduced Latin squares differ from each other by a permutation of their symbols. Two such Latin squares that differ by a permutation of their symbols have been called isoplanar by Bailey (1984), cited by Nilrat and Praeger (1988), cited by Denes and Keedwell (1991). Latin squares based on dihedral groups are of interest in the stable marriage problem, where Benjamin et al. (1995) exhibited such squares having many stable matchings when viewed as ranking matrices. Two isoplanar Latin squares generally produce a different number of stable matchings, so there is motivation to generate all symbol permutations to find the ones with the most stable matchings.
See comments in A002618 regarding automorphisms of dihedral groups by Ola Veshta and Yaghoub Sharifi. - Dan Eilers, Jun 08 2024

Examples

			For n=3 the a(3)=20 isoplanar reduced Latin squares based on the dihedral group of order 6, in lexicographical order, are:
1)             2)             3)             4)             5)
1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6
2 1 4 3 6 5    2 1 4 3 6 5    2 1 4 3 6 5    2 1 4 3 6 5    2 1 5 6 3 4
3 5 1 6 2 4    3 5 6 2 4 1    3 6 1 5 4 2    3 6 5 2 1 4    3 4 1 2 6 5
4 6 2 5 1 3    4 6 5 1 3 2    4 5 2 6 3 1    4 5 6 1 2 3    4 3 6 5 1 2
5 3 6 1 4 2    5 3 2 6 1 4    5 4 6 2 1 3    5 4 1 6 3 2    5 6 2 1 4 3
6 4 5 2 3 1    6 4 1 5 2 3    6 3 5 1 2 4    6 3 2 5 4 1    6 5 4 3 2 1
6)             7)             8)             9)             10)
1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6
2 1 5 6 3 4    2 1 5 6 3 4    2 1 5 6 3 4    2 1 6 5 4 3    2 1 6 5 4 3
3 4 6 5 2 1    3 6 1 5 4 2    3 6 4 1 2 5    3 4 1 2 6 5    3 4 5 6 1 2
4 3 2 1 6 5    4 5 6 1 2 3    4 5 1 3 6 2    4 3 5 6 2 1    4 3 2 1 6 5
5 6 4 3 1 2    5 4 2 3 6 1    5 4 6 2 1 3    5 6 4 3 1 2    5 6 1 2 3 4
6 5 1 2 4 3    6 3 4 2 1 5    6 3 2 5 4 1    6 5 2 1 3 4    6 5 4 3 2 1
11)            12)            13)            14)            15)
1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6
2 1 6 5 4 3    2 1 6 5 4 3    2 3 1 5 6 4    2 3 1 6 4 5    2 4 5 1 6 3
3 5 1 6 2 4    3 5 4 1 6 2    3 1 2 6 4 5    3 1 2 5 6 4    3 6 1 5 4 2
4 6 5 1 3 2    4 6 1 3 2 5    4 6 5 1 3 2    4 5 6 1 2 3    4 1 6 2 3 5
5 3 4 2 6 1    5 3 2 6 1 4    5 4 6 2 1 3    5 6 4 3 1 2    5 3 2 6 1 4
6 4 2 3 1 5    6 4 5 2 3 1    6 5 4 3 2 1    6 4 5 2 3 1    6 5 4 3 2 1
16)            17)            18)            19)            20)
1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6    1 2 3 4 5 6
2 4 6 1 3 5    2 5 4 6 1 3    2 5 6 3 1 4    2 6 4 5 3 1    2 6 5 3 4 1
3 5 1 6 2 4    3 6 1 5 4 2    3 4 1 2 6 5    3 5 1 6 2 4    3 4 1 2 6 5
4 1 5 2 6 3    4 3 2 1 6 5    4 6 5 1 3 2    4 3 2 1 6 5    4 5 6 1 2 3
5 6 4 3 1 2    5 1 6 3 2 4    5 1 4 6 2 3    5 4 6 2 1 3    5 3 2 6 1 4
6 3 2 5 4 1    6 4 5 2 3 1    6 3 2 5 4 1    6 1 5 3 4 2    6 1 4 5 3 2
		

References

  • Denes, J. and Keedwell, A. D. (1991) Latin Squares New Developments in the Theory and Applications. p. 98.

Crossrefs

Cf. A058163 (all groups), A058162 (Abelian groups), A058161 (cyclic groups), A069156 (stable matchings), A002618 (n*phi(n)).

Programs

  • GAP
    A336412:=List([1..16], n->Factorial(2*n-1)/Size(AutomorphismGroup(DihedralGroup(2*n)))); # Dan Eilers, Jun 08 2024

Formula

a(1) = a(2) = 1; a(n>2) = (2*n-1)! / A002618(n). - Dan Eilers, Jun 08 2024

Extensions

a(8)-a(16) and edited by Dan Eilers, Jun 08 2024
Showing 1-7 of 7 results.