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A205337
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Number of length n+1 nonnegative integer arrays starting and ending with 0 with adjacent elements unequal but differing by no more than 4.
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2
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0, 4, 12, 82, 454, 2912, 18652, 124299, 841400, 5800725, 40506816, 286137616, 2040430976, 14670243774, 106225269954, 773958961125, 5670067999156, 41742291894425, 308645064367896, 2291123920091484, 17067970534656790
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OFFSET
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1,2
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COMMENTS
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Number of excursions (walks starting at the origin, ending on the x-axis, and never go below the x-axis in between) with n steps from {-4,-3,-2,-1,1,2,3,4}. - David Nguyen, Dec 20 2016
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LINKS
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FORMULA
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a(n) = Sum_{i=1..n}((Sum_{l=0..i}(binomial(i,l)*(Sum_{j=0..(4*(i-l))/9}((-1)^j*binomial(i-l,j)*binomial(-l+4*(-l-2*j+i)-j+i-1,4*(-l-2*j+i)-j)))*(-1)^l))*a(n-i))/n, a(0)=1. - Vladimir Kruchinin, Apr 07 2017
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EXAMPLE
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Some solutions for n=5
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
..2....3....2....4....4....4....1....2....4....3....3....1....2....3....2....4
..3....5....6....3....0....5....0....4....6....1....5....0....3....1....0....2
..6....1....2....2....1....3....3....6....3....4....3....1....6....2....1....5
..2....2....1....1....3....4....1....4....4....2....4....2....4....3....4....2
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
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MATHEMATICA
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a[n_] := a[n] = If[n == 0, 1, Sum[Sum[Binomial[i, l] Sum[(-1)^j Binomial[i - l, j] Binomial[-l + 4(-l - 2j + i) - j + i - 1, 4(-l - 2j + i) - j], {j, 0, (4(i - l))/9}] (-1)^l, {l, 0, i}] a[n - i], {i, 1, n}]/n];
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PROG
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(Maxima)
a(n):=if n=0 then 1 else sum(sum(binomial(i, l)*sum((-1)^j*binomial(i-l, j)*binomial(-l+4*(-l-2*j+i)-j+i-1, 4*(-l-2*j+i)-j), j, 0, (4*(i-l))/9)*(-1)^l, l, 0, i)*a(n-i), i, 1, n)/n; /* Vladimir Kruchinin, Apr 07 2017 */
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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