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 A055561 Numbers n such that there are precisely 3 groups of order n. 25
 75, 363, 609, 867, 1183, 1265, 1275, 1491, 1587, 1725, 1805, 2067, 2175, 2373, 2523, 3045, 3525, 3685, 3795, 3975, 4137, 4205, 4335, 4425, 4895, 5019, 5043, 5109, 5901, 5915, 6171, 6225, 6627, 6675, 6699, 7935, 8025, 8427, 8475, 8855, 9429, 9537, 10275 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Let gnu(n) (= A000001(n)) denote the "group number of n" defined in A000001 or in (J. H. Conway, Heiko Dietrich and E. A. O'Brien, 2008), then the sequence n -> gnu(a(n)) -> gnu(gnu(a(n))) consists of 1's. - Muniru A Asiru, Nov 19 2017 LINKS Gheorghe Coserea, Table of n, a(n) for n = 1..234567, terms 1..206 from Muniru A Asiru. H.-U. Besche, B. Eick and E. A. O'Brien, The Small Groups Library H.-U. Besche, B. Eick and E. A. O'Brien, A Millennium Project: Constructing Small Groups, Internat. J. Algebra and Computation, 12 (2002), 623-644. J. H. Conway, Heiko Dietrich and E. A. O'Brien, Counting groups: gnus, moas and other exotica, Math. Intell., Vol. 30, No. 2, Spring 2008. Gordon Royle, Numbers of Small Groups EXAMPLE For n = 75, the 3 groups of order 75 are C75, (C5 x C5) : C3, C15 x C5 and for n = 363 the 3 groups of order 363 are C363, (C11 x C11) : C3, C33 x C11 where C is the Cyclic group of the stated order. The symbols x and : mean direct and semi-direct products respectively. - Muniru A Asiru, Oct 24 2017 PROG (PARI) is(n) = {   my(p = gcd(n, eulerphi(n)), f, g);   if (isprime(p), return(n % p^2 == 0 && isprime(gcd(p+1, n))));   if (omega(p) != 2 || !issquarefree(n), return(0));   f = factor(n); g = factor(p);   1 == g[2, 1] % g[1, 1] &&   1 == sum(k=1, matsize(f)[1], f[k, 1] % g[1, 1] == 1) &&   1 == sum(k=1, matsize(f)[1], f[k, 1] % g[2, 1] == 1); }; seq(N) = {   my(a = vector(N), k=0, n=1);   while(k < N, if(is(n), a[k++]=n); n++); a; }; seq(43) \\ Gheorghe Coserea, Dec 12 2017 CROSSREFS Cf. A000001, A003277, A054395, A054396, A054397. Sequence in context: A158765 A226741 A223078 * A193252 A223452 A015223 Adjacent sequences:  A055558 A055559 A055560 * A055562 A055563 A055564 KEYWORD nonn AUTHOR Christian G. Bower, May 25 2000; Nov 12 2003; Feb 17 2006 STATUS approved

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Last modified September 21 11:46 EDT 2020. Contains 337268 sequences. (Running on oeis4.)