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A206464
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Number of length-n Catalan-RGS (restricted growth strings) such that the RGS is a valid mixed-radix number in falling factorial basis.
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4
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1, 1, 2, 4, 10, 26, 74, 218, 672, 2126, 6908, 22876, 77100, 263514, 911992, 3189762, 11261448, 40083806, 143713968, 518594034, 1882217168, 6867064856, 25172021144, 92666294090, 342467464612, 1270183943200, 4726473541216, 17640820790092, 66025467919972
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OFFSET
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0,3
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COMMENTS
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Catalan-RGS are strings with first digit d(0)=zero, and d(k+1) <= d(k)+1, falling factorial mixed-radix numbers have last digit <= 1, second last <= 2, etc.
The digits of the RGS are <= floor(n/2).
The first few terms are the same as for A089429.
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LINKS
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FORMULA
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Conjecture: a(n) = Sum_{k = 0..floor(n/4)} (-1)^k * C(floor(n/2) + 1 - k, k + 1) * a(n - 1 - k), a(0) = 1. - Gionata Neri, Jun 17 2018
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EXAMPLE
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The a(5)=26 strings for n=5 are (dots for zeros):
1: [ . . . . . ]
2: [ . . . . 1 ]
3: [ . . . 1 . ]
4: [ . . . 1 1 ]
5: [ . . 1 . . ]
6: [ . . 1 . 1 ]
7: [ . . 1 1 . ]
8: [ . . 1 1 1 ]
9: [ . . 1 2 . ]
10: [ . . 1 2 1 ]
11: [ . 1 . . . ]
12: [ . 1 . . 1 ]
13: [ . 1 . 1 . ]
14: [ . 1 . 1 1 ]
15: [ . 1 1 . . ]
16: [ . 1 1 . 1 ]
17: [ . 1 1 1 . ]
18: [ . 1 1 1 1 ]
19: [ . 1 1 2 . ]
20: [ . 1 1 2 1 ]
21: [ . 1 2 . . ]
22: [ . 1 2 . 1 ]
23: [ . 1 2 1 . ]
24: [ . 1 2 1 1 ]
25: [ . 1 2 2 . ]
26: [ . 1 2 2 1 ]
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MAPLE
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b:= proc(i, l) option remember;
`if`(i<=0, 1, add(b(i-1, j), j=0..min(l+1, i)))
end:
a:= n-> b(n-1, 0):
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MATHEMATICA
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b[i_, l_] := b[i, l] = If[i <= 0, 1, Sum[b[i-1, j], {j, 0, Min[l+1, i]}]];
a[n_] := b[n-1, 0];
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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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