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A054395 Numbers n such that there are precisely 2 groups of order n. 24
4, 6, 9, 10, 14, 21, 22, 25, 26, 34, 38, 39, 45, 46, 49, 55, 57, 58, 62, 74, 82, 86, 93, 94, 99, 105, 106, 111, 118, 121, 122, 129, 134, 142, 146, 153, 155, 158, 165, 166, 169, 175, 178, 183, 194, 195, 201, 202, 203, 205, 206, 207, 214, 218, 219, 226, 231, 237 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Givens characterizes this sequence, see Theorem 5. In particular, this sequence is ({n: A215935(n) = 1} INTERSECT A005117) UNION (A060687 INTERSECT A051532). - Charles R Greathouse IV, Aug 27 2012

Numbers n such that A000001(n) = 2. - Muniru A Asiru, Nov 03 2017

LINKS

Muniru A Asiru and Gheorghe Coserea, Table of n, a(n) for n = 1..234567, terms 1..422 from Muniru A Asiru.

H. U. Besche, B. Eick and E. A. O'Brien, The Small Groups Library

Clint Givens, Orders for which there exist exactly two groups (2006)

Gordon Royle, Numbers of Small Groups

Index entries for sequences related to groups

EXAMPLE

For n = 4, the 2 groups of order 4 are C4, C2 x C2, for n = 6, the 2 groups of order 6 are S3, C6 and for n = 9, the 2 groups of order 9 are C9, C3 x C3 where C is the Cyclic group of the stated order and S is the Symmetric group of the stated degree. The symbol x means direct product. - Muniru A Asiru, Oct 24 2017

MATHEMATICA

Select[Range[240], FiniteGroupCount[#] == 2&]

(* or: *)

okQ[n_] := Module[{p, f}, p = GCD[n, EulerPhi[n]]; If[! PrimeQ[p], Return[False]]; If[Mod[n, p^2] == 0, Return[1 == GCD[p + 1, n]]]; f = FactorInteger[n]; 1 == Sum[Boole[Mod[f[[k, 1]], p] == 1], {k, 1, Length[f]}]];

Select[Range[240], okQ] (* Jean-Fran├žois Alcover, Dec 08 2017, after Gheorghe Coserea *)

PROG

(GAP) A054395 := Filtered([1..2015], n -> NumberSmallGroups(n) = 2); # Muniru A Asiru, Oct 24 2017

(GAP)

IsGivensInt := function(n)

  local p, f; p := GcdInt(n, Phi(n));

  if not IsPrimeInt(p) then return false; fi;

  if n mod p^2 = 0 then return 1 = GcdInt(p+1, n); fi;

  f := PrimePowersInt(n);

  return 1 = Number([1..QuoInt(Length(f), 2)], k->f[2*k-1] mod p = 1);

end;;

Filtered([1..240], IsGivensInt); # Gheorghe Coserea, Dec 04 2017

(PARI)

is(n) = {

  my(p=gcd(n, eulerphi(n)), f);

  if (!isprime(p), return(0));

  if (n%p^2 == 0, return(1 == gcd(p+1, n)));

  f = factor(n); 1 == sum(k=1, matsize(f)[1], f[k, 1]%p==1);

};

seq(N) = {

  my(a = vector(N), k=0, n=1);

  while(k < N, if(is(n), a[k++]=n); n++); a;

};

seq(58) \\ Gheorghe Coserea, Dec 03 2017

CROSSREFS

Cf. A000001, A003277, A054395, A054396, A054397, A055561, A135850.

Sequence in context: A236026 A193305 A084759 * A142863 A318990 A132435

Adjacent sequences:  A054392 A054393 A054394 * A054396 A054397 A054398

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, May 21 2000

EXTENSIONS

More terms from Christian G. Bower, May 25 2000

STATUS

approved

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Last modified February 23 09:04 EST 2019. Contains 320420 sequences. (Running on oeis4.)