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Search: id:A088456
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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
1
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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
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OFFSET
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0,3
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EXAMPLE
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[1,1,1],[1,1,2],[1,2,2],[1,2,3] are the a(3)=4 sequences.
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PROGRAM
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(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
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Adjacent sequences: A088453 A088454 A088455 this_sequence A088457 A088458 A088459
Sequence in context: A115324 A107092 A055588 this_sequence A091561 A025265 A037245
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KEYWORD
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nonn
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AUTHOR
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Michael Somos, Oct 01 2003
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