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A026030
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a(n) = T(2n,n-1), where T is defined in A026022.
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0
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1, 4, 15, 56, 209, 780, 2912, 10880, 40698, 152456, 572033, 2150040, 8095425, 30535260, 115377660, 436698240, 1655607390, 6286707000, 23908446510, 91057063344, 347281885818, 1326262602104, 5071418015120, 19415851639296, 74419447792340
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| a(n) = number of (s(0), s(1), ..., s(2n)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| = 1 for i = 1,2,...,n, s(0) = 3, s(2n) = 5.
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FORMULA
| a(n) = C(2n, n-1) - C(2n, n-5). G.f.: (1+x^2C^4)*C^4, where C=(1-sqrt(1-4x))/(2x). - Ralf Stephan, Jan 09 2005
G.f.: 2*x*(1-2*x) / ((1-2*x)*(1-4*x+x^2) + (1-x)*(1-3*x)*sqrt(1-4*x)). - Michael Somos, Jan 08 2012
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EXAMPLE
| x + 4*x^2 + 15*x^3 + 56*x^4 + 209*x^5 + 780*x^6 + 2912*x^7 + 10880*x^8 + ...
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PROG
| (PARI) {a(n) = binomial( 2*n, n-1) - binomial( 2*n, n-5)} /* Michael Somos, Jan 08 2012 */
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CROSSREFS
| Cf. A001075.
Sequence in context: A106707 A125905 A195503 * A047038 A158500 A001791
Adjacent sequences: A026027 A026028 A026029 * A026031 A026032 A026033
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KEYWORD
| nonn
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AUTHOR
| Clark Kimberling (ck6(AT)evansville.edu)
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