A350332 - OEIS

%I #18 Dec 25 2021 15:00:37

%S 45,99,153,175,207,261,325,369,423,425,475,477,531,539,575,637,639,

%T 725,747,801,833,909,925,931,963,1017,1075,1127,1175,1179,1233,1325,

%U 1341,1475,1503,1519,1557,1573,1611,1675,1719,1773,1813,1825,1975,2009,2043,2057

%N Numbers p^2*q, p < q odd primes such that p does not divide q-1.

%C For these terms m, there are precisely 2 groups of order m, so this is a subsequence of A054395.

%C The 2 groups are abelian; they are C_{p^2*q} and (C_p X C_p) X C_q, where C means cyclic groups of the stated order and the symbol X means direct product.

%D Pascal Ortiz, Exercices d'Algèbre, Collection CAPES / Agrégation, Ellipses, problème 1.35, pp. 70-74, 2004.

%e 99 = 3^2 * 11, 3 and 11 are odd and 3 does not divide 11-1 = 10, hence 99 is a term.

%e 175 = 5^2 * 7, 5 and 7 are odd and 5 does not divide 7-1 = 6, hence 115 is another term.

%t q[n_] := Module[{f = FactorInteger[n], p, e}, p = f[[;; , 1]]; e = f[[;; , 2]]; e == {2, 1} && ! Divisible[p[[2]] - 1, p[[1]]]]; Select[Range[2000], q] (* _Amiram Eldar_, Dec 25 2021 *)

%o (Python)

%o from sympy import integer_nthroot, primerange

%o def aupto(limit):

%o     aset, maxp = set(), integer_nthroot(limit, 3)[0]

%o     for p in primerange(3, maxp+1):

%o         pp = p*p

%o         for q in primerange(p+1, limit//pp+1):

%o             if (q-1)%p != 0:

%o                 aset.add(pp*q)

%o     return sorted(aset)

%o print(aupto(2060)) # _Michael S. Branicky_, Dec 25 2021

%o (PARI) isok(m) = my(f=factor(m)); if (f[, 2] == [2, 1]~, my(p=f[1, 1], q=f[2, 1]); ((q-1) % p)); \\ _Michel Marcus_, Dec 25 2021

%Y Subsequence of A051532, A054395, A054753 and of A060687.

%Y Other subsequences of A054753 linked with order of groups: A079704, A143928, A349495, A350115, A350245.

%K nonn

%O 1,1

%A _Bernard Schott_, Dec 25 2021

%E More terms from _Michael S. Branicky_, Dec 25 2021