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 A192005 Number of non-cyclic abelian groups of finite order. The order is given by A013929. 3
 1, 2, 1, 1, 4, 1, 1, 2, 1, 2, 1, 6, 3, 2, 1, 1, 4, 1, 1, 1, 2, 2, 1, 1, 10, 1, 5, 1, 1, 4, 4, 1, 2, 1, 1, 6, 1, 1, 3, 2, 5, 4, 1, 1, 2, 1, 1, 2, 1, 14, 1, 2, 2, 1, 9, 1, 1, 1, 2, 1, 1, 6, 4, 1, 2, 1, 1, 1, 1, 4, 3, 2, 1, 2, 10, 3, 1, 5, 1, 1, 4, 1, 8, 1, 6, 3, 1, 2, 1, 1, 4, 1, 6, 1, 1, 2, 2, 3, 21, 1, 1, 2, 1, 2, 4, 1, 1, 1, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Every abelian group of finite order is the direct product of cyclic groups (there may be only one factor). See, e.g., the A. Speiser reference, Satz 43, p. 49, in combination with Satz 42, p. 47, and also Satz 4, p. 17, with the remark on the direct product on page 28. See the list of abelian groups of small order in the Wikipedia link. REFERENCES A. Speiser, Die Theorie der Gruppen von endlicher Ordnung, Vierte Auflage, 1956, BirkhĂ¤user. LINKS Wikipedia, List of small groups. FORMULA a(n) = A000688(A013929(n)) - 1, n>=1. See the formula for A000688 using the product of the number of partitions of the exponents in the prime number factorization. EXAMPLE n=1: there is one abelian group of order 4=A013929(1), which is not the cyclic group Z_4 (in additive notation), namely the Klein 4-group: Z_2 x Z_2, (also denoted by (Z_2)^2. n=2: there are 2 non-cyclic abelian groups of order 8=A013929(2), namely Z_2 x Z_4 and (Z_2)^3. n=3: order 9=A013929(3), (Z_3)^2. n=4: order 12, Z_3 x (Z_2)^2 (note that Z_6 = Z_3 x Z_2 and Z_12 = Z_4 x Z_3, where = means 'is isomorphic to'). n=5: order 16. The four non-cyclic groups are (Z_2)^4, Z_4 x (Z_2)^2, Z_8 x Z_2 and (Z_4)^2. CROSSREFS Cf. A000688, A013929. Sequence in context: A285425 A008307 A249694 * A333810 A205592 A099238 Adjacent sequences:  A192002 A192003 A192004 * A192006 A192007 A192008 KEYWORD nonn,easy AUTHOR Wolfdieter Lang, Jul 28 2011 STATUS approved

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Last modified May 26 17:02 EDT 2020. Contains 334630 sequences. (Running on oeis4.)