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A060823
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4-wave sequence beginning with 2's with middles dropped.
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1
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2, 2, 8, 20, 60, 170, 492, 1414, 4074, 11728, 33772, 97240, 279994, 806208, 2321386, 6684162, 19246280, 55417452, 159568196, 459458306, 1322957468, 3809304206, 10968454314, 31582405472, 90937912212, 261845282320, 753953441490
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OFFSET
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2,1
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LINKS
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FORMULA
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a(n) = 2*a(n-1) + 3*a(n-2) - a(n-3) - a(n-4). - Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Mar 22 2003
G.f.: -2*x^2*(-1+x+x^2)/((1+x)*(x^3-3*x+1)).
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MATHEMATICA
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LinearRecurrence[{2, 3, -1, -1}, {2, 2, 8, 20}, 30] (* Harvey P. Dale, Aug 31 2016 *)
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PROG
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(PARI) { for (n=2, 200, if (n>5, a=2*a1 + 3*a2 - a3 - a4; a4=a3; a3=a2; a2=a1; a1=a, if (n==2, a=a4=2, if (n==3, a=a3=2, if (n==4, a=a2=8, a=a1=20)))); write("b060823.txt", n, " ", a); ) } \\ Harry J. Smith, Jul 12 2009
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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