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A088456
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Number of integer sequences [y(1),...,y(n)] such that |y(i)-y(i+1)|<=1, 1=y(1)<=y(i)<=y(n).
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0
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1, 1, 2, 4, 9, 22, 56, 146, 387, 1039, 2819, 7717, 21287, 59105, 165038, 463089, 1304945, 3690970, 10474213, 29811119, 85070660, 243340754, 697571153, 2003636629, 5765473829, 16617825248, 47971281916, 138677926537
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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EXAMPLE
| [1,1,1],[1,1,2],[1,2,2],[1,2,3] are the a(3)=4 sequences.
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PROG
| (PARI) a(n)=local(p0, p1, p2); if(n<0, 0, p1=1; polcoeff(sum(i=0, n, if(p2=(1-x)*p1-x^2*p0, p0=p1; p1=p2; x^i/p0), x*O(x^n)), n))
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CROSSREFS
| Sequence in context: A196307 A107092 A055588 * A152225 A091561 A025265
Adjacent sequences: A088453 A088454 A088455 * A088457 A088458 A088459
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KEYWORD
| nonn
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AUTHOR
| Michael Somos, Oct 01 2003
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