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A093374
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Number of 1-2-3-avoiding permutations with exactly thrice the 1-3-2 pattern.
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0
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1, 5, 18, 57, 168, 472, 1280, 3376, 8704, 22016, 54784, 134400, 325632, 780288, 1851392, 4354048, 10158080, 23527424, 54132736, 123797504, 281542656, 637009920, 1434451968, 3215982592, 7180648448, 15971909632, 35399925760
(list; graph; refs; listen; history; internal format)
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OFFSET
| 4,2
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LINKS
| D. Callan, A recursive bijective approach to counting permutations...
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FORMULA
| a(n) = C(n-3, 1)2^(n-4) + C(n-3, 1)2^(n-5) + C(n-3, 2)2^(n-7); for n<4, a(n) = 0.
G.f.: x^3(1-3x+2x^2+x^3)/(1-2x)^4.
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PROG
| (PARI) a(n)=if(n<4, 0, 2^(n-4)*binomial(n-3, 1)+2^(n-5)*binomial(n-3, 2)+2^(n-7)*binomial(n-4, 3))
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CROSSREFS
| Sequence in context: A099450 A145129 A001793 * A000745 A128553 A190163
Adjacent sequences: A093371 A093372 A093373 * A093375 A093376 A093377
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KEYWORD
| nonn
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AUTHOR
| Ralf Stephan, Apr 28 2004
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