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A121704
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Number of separable involutions.
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8
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1, 2, 4, 10, 24, 64, 166, 456, 1234, 3454, 9600, 27246, 77132, 221336, 635078, 1839000, 5331274, 15555586, 45465412, 133517130, 392841336, 1160033656, 3432015726, 10182891552, 30267591290, 90177226062, 269117947728, 804699330974, 2409839825756, 7228746487536
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OFFSET
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1,2
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COMMENTS
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The separable permutations are those avoiding 2413 and 3142 and are counted by the large Schroeder numbers (A006318).
The involutions avoiding 2413 coincide with the involutions avoiding 3142, and hence both sets coincide with the separable involutions. - David Callan, Aug 27 2014
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LINKS
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FORMULA
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G.f. f satisfies: x^2f^4 + (x^3+3x^2+x-1)f^3 + (3x^3+6x^2-x)f^2 + (3x^3+7x^2-x-1)f +x^3+3x^2+x=0.
a(n) ~ sqrt(6 + 6*sqrt(2) + 4*sqrt(3) + 3*sqrt(6)) * (5+2*sqrt(6))^(n/2) / (2 * sqrt(Pi) * n^(3/2)). - Vaclav Kotesovec, Sep 13 2014
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EXAMPLE
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a(5) = 24 because of the 26 involutions of length 5 only two are not separable, 35142 and 42513.
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MATHEMATICA
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terms = 30;
f[_] = 0; Do[f[x_] = Normal[(-(x^3 f[x]^3) - 3 x^3 f[x]^2 - x^2 f[x]^4 - 3 x^2 f[x]^3 - 6 x^2 f[x]^2 - x f[x]^3 + f[x]^3 + x f[x]^2 - x^3 - 3 x^2 - x)/(3 x^3 + 7 x^2 - x - 1) + O[x]^(terms+1)], {terms+1}];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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