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 A230439 Number of contractible "tight" meanders of width n. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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_00 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; MATHEMATICA (* 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]]     ]    ] CROSSREFS For various kinds of meandric numbers see A005315, A005316, A060066, A060089, A060206. Sequence in context: A005380 A257557 A124612 * A184697 A124613 A296626 Adjacent sequences:  A230436 A230437 A230438 * A230440 A230441 A230442 KEYWORD nonn AUTHOR Mamuka Jibladze, Nov 04 2013 STATUS approved

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Last modified July 22 20:51 EDT 2019. Contains 325226 sequences. (Running on oeis4.)