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A024430
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Expansion of e.g.f. cosh(exp(x)-1).
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32
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1, 0, 1, 3, 8, 25, 97, 434, 2095, 10707, 58194, 338195, 2097933, 13796952, 95504749, 692462671, 5245040408, 41436754261, 340899165549, 2915100624274, 25857170687507, 237448494222575, 2253720620740362, 22078799199129799, 222987346441156585, 2319210969809731600
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OFFSET
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0,4
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COMMENTS
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Number of partitions of an n-element set into an even number of classes.
Let A(0) = 1, B(0) = 0; A(n+1) = Sum_{k=0..n} binomial(n,k)*B(k), B(n+1) = Sum_{k=0..n} binomial(n,k)*A(k); entry gives A sequence (cf. A024429).
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REFERENCES
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L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 226, 5th line of table.
S. K. Ghosal, J. K. Mandal, Stirling Transform Based Color Image Authentication, Procedia Technology, 2013 Volume 10, 2013, Pages 95-104.
L. Lovasz, Combinatorial Problems and Exercises, North-Holland, 1993, pp. 15, 148.
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LINKS
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FORMULA
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a(n) = S(n, 2) + S(n, 4) + ... + S(n, 2k), where k = [ n/2 ], S(i, j) are Stirling numbers of second kind.
O.g.f.: Sum_{n>=0} x^(2*n) / Product_{k=0..2*n} (1 - k*x). - Paul D. Hanna, Sep 05 2012
G.f.: G(0)/(1+x) where G(k) = 1 - x*(2*k+1)/((2*x*k-1) - x*(2*x*k-1)/(x - (2*k+1)*(2*x*k+x-1)/G(k+1) )); (recursively defined continued fraction). - Sergei N. Gladkovskii, Jan 05 2013
G.f.: G(0)/(1+2*x) where G(k) = 1 - 2*x*(k+1)/((2*x*k-1) - x*(2*x*k-1)/(x - 2*(k+1)*(2*x*k+x-1)/G(k+1) )); (recursively defined continued fraction). - Sergei N. Gladkovskii, Jan 05 2013
a(n) ~ n^n / (2 * (LambertW(n))^n * exp(n+1-n/LambertW(n)) * sqrt(1+LambertW(n))). - Vaclav Kotesovec, Aug 04 2014
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MAPLE
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b:= proc(n, t) option remember; `if`(n=0, t, add(
b(n-j, 1-t)*binomial(n-1, j-1), j=1..n))
end:
a:= n-> b(n, 1):
with(combinat); seq((bell(n) + BellB(n, -1))/2, n = 0..20); # G. C. Greubel, Oct 09 2019
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MATHEMATICA
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nn=20; a=Exp[Exp[x]-1]; Range[0, nn]!CoefficientList[Series[(a+1/a)/2, {x, 0, nn}], x] (* Geoffrey Critzer, Nov 04 2012 *)
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PROG
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(Sage)
return add(stirling_number2(n, i) for i in range(0, n+(n+1)%2, 2))
(PARI) {a(n)=polcoeff(sum(m=0, n, x^(2*m)/prod(k=1, 2*m, 1-k*x +x*O(x^n))), n)} \\ Paul D. Hanna, Sep 05 2012
(Magma) a:= func< n | (&+[StirlingSecond(n, 2*k): k in [0..Floor(n/2)]]) >;
(GAP) List([0..25], n-> Sum([0..Int(n/2)], k-> Stirling2(n, 2*k)) ); # G. C. Greubel, Oct 09 2019
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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