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A007359
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Number of partitions of n into pairwise coprime parts that are >= 2.
(Formerly M0143)
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70
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1, 0, 1, 1, 1, 2, 1, 3, 2, 3, 3, 5, 4, 6, 5, 5, 8, 9, 10, 11, 11, 10, 14, 18, 19, 18, 20, 20, 25, 30, 35, 34, 32, 32, 43, 43, 57, 56, 51, 55, 67, 78, 87, 87, 80, 82, 97, 125, 128, 127, 128, 127, 146, 182, 191, 185, 184, 193, 213, 263, 290, 279, 258, 271, 312, 354, 404, 402
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OFFSET
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0,6
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COMMENTS
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This sequence is of interest for group theory. The partitions counted by a(n) correspond to conjugacy classes of optimal order of the symmetric group of n elements: they have no fixed point, their order is the direct product of their cycle lengths and they are not contained in a subgroup of Sym_p for p < n. A123131 gives the maximum order (LCM) reachable by these partitions.
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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EXAMPLE
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The a(17) = 9 strict partitions into pairwise coprime parts that are greater than 1 are (17), (15,2), (14,3), (13,4), (12,5), (11,6), (10,7), (9,8), (7,5,3,2). - Gus Wiseman, Apr 14 2018
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MAPLE
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with(numtheory):
b:= proc(n, i, s) option remember; local f;
if n=0 then 1
elif i<2 then 0
else f:= factorset(i);
b(n, i-1, select(x-> is(x<i), s)) +`if`(i<=n and f intersect s={},
b(n-i, i-1, select(x-> is(x<i), s union f)), 0)
fi
end:
a:= n-> b(n, n, {}):
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MATHEMATICA
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b[n_, i_, s_] := b[n, i, s] = Module[{f}, If[n == 0 || i == 1, 1, If[i<2, 0, f = FactorInteger[i][[All, 1]]; b[n, i-1, Select[s, #<i&]]+If[i <= n && f ~Intersection~ s == {}, b[n-i, i-1, Select[s ~Union~ f, #<i&]], 0]]]]; a[n_] := b[n, n, {}]-b[n-1, n-1, {}]; Table[a[n], {n, 0, 80}] (* Jean-François Alcover, Feb 17 2014, after Alois P. Heinz *)
Table[Length[Select[IntegerPartitions[n], FreeQ[#, 1]&&(Length[#]===1||CoprimeQ@@#)&]], {n, 20}] (* Gus Wiseman, Apr 14 2018 *)
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CROSSREFS
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Cf. A000837, A007359, A007360, A051424, A101268, A123131, A184956, A187718, A289508, A289509, A298748, A302569, A302696, A302698, A302797.
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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More terms from Pab Ter (pabrlos2(AT)yahoo.com), Nov 13 2005
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STATUS
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approved
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