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A230439
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Number of contractible "tight" meanders of width n.
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1
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1, 2, 6, 14, 34, 68, 150, 296, 586, 1140, 2182, 4130, 7678, 14368, 26068, 48248, 86572, 158146, 281410, 509442, 901014, 1618544, 2852464, 5089580, 8948694, 15884762, 27882762, 49291952, 86435358, 152316976, 266907560, 469232204, 821844316
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OFFSET
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1,2
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COMMENTS
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A tight meander of width n is a special kind of meander defined as follows.
For any pair (S={s_1,...,s_k},T={t_1,...,t_l}) of subsets of {1,...,n-1} (k or l might be 0), the tight meander M(S,T) defined by (S,T) is the following subset of R^2:
assuming S and T ordered so that 0=s_0<s_1<...<s_k<s_{k+1}=n and 0=t_0<t_1<...<t_l<t_{l+1}=n, let M(S,T) be the union of the set of points {(1,0),...,(n,0)},
semicircles in the upper half-plane with endpoints (s_{i-1}+j,0) and (s_i+1-j,0), for i=1,...,k+1, and j positive integer with s_{i-1}+j<s_i+1-j,
and semicircles in the lower half-plane with endpoints (t_{i-1}+j,0) and (t_i+1-j,0), for i=1,...,l+1, and j positive integer with t_{i-1}+j<t_i+1-j.
The tight meander M(S,T) is called contractible if it is a contractible subspace of R^2, i.e., is either a single point or homeomorphic to an interval.
Then, a(n) is the number of pairs (S,T) as above such that the tight meander M(S,T) is contractible.
The following is a definition for closed meanders that yield the same sequence as tight meanders. T(n,k) = the number of closed meanders with n top arches and with k exterior arches and k arches of length 1.
e = exterior arch (arch with no covering arch), 1 = arch with length 1, e1 = arch that is exterior with a length of 1:
e exterior length 1
____________ arches arches
/ ______ \
e1 / / \ \ top = 2 top = 2
/\ / / /\1 \ \
/ \ / / / \ \ \
\ \ / / \ \ / / bottom = 2 bottom = 2
\ \/1 / \ \/1 / total = 4 total = 4
\______/ \______/
e e Example T(4,4).
(End)
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LINKS
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EXAMPLE
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For n=3 the a(3)=6 contractible tight meanders of width 3 correspond to the following pairs of subsets of {1,2}: ({},{1}), ({},{2}), ({1},{}), ({2},{}), ({1},{2}), ({2},{1}).
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MAPLE
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# The Maple program based on the C code by Martin Plechsmid
proc()
local n, a, b, d, r;
option remember;
if args[1]=1 then
1
elif nargs=1 then
2*`+`(''procname(args, [i], [j])'$'j'=1..i-1'$'i'=2..args)
else
n:=args[1]; a:=args[2]; b:=args[3];
if b=[] then
`+`('procname(n, a, [k])'$'k'=1..n)
elif a[1]=b[1] then
0
elif a[1]<b[1] then
procname(n, b, a)
else
d:=a[1]-b[1];
r:=irem(b[1], d);
if r>0 then
procname(n-b[1], [d-r, op(subsop(1=r, a))], subsop(1=NULL, b))
else
procname(n-b[1], subsop(1=d, a), subsop(1=NULL, b))
fi
fi
fi
end;
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MATHEMATICA
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(* Mathematica program based on the C code by Martin Plechsmid *)
f[n_, a___, b___] := f[n, a, b] =
Which[
n == 1, 1,
a == Null, 2 Sum[f[n, {i}, {j}], {i, 2, n}, {j, i - 1}],
b == {}, Sum[f[n, a, {i}], {i, n}],
First[a] == First[b], 0,
First[a] < First[b], f[n, b, a],
True,
Block[{d = First[a] - First[b], r, s},
r = Mod[First[b], d];
s = If[r == 0, {d}, {d - r, r}];
f[n - First[b], Join[s, Rest[a]], Rest[b]]
]
]
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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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