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A183757 Unlabeled super-Catalan numbers: patterns of nonintersecting chords joining unlabeled points on a circle 2
1, 1, 2, 6, 17, 74, 324, 1558, 7640, 38245 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Interpret the super-Catalan sequence (A001003) as follows:  Ways to insert parentheses in a string of n+1 symbols. The parentheses must be balanced but there is no restriction on the number of pairs of parentheses. The number of letters inside a pair of parentheses must be at least 2. Parentheses enclosing the whole string are ignored.  Now picture the x's and parentheses as equally spaced unlabeled points on a circle with chords joining paired parentheses and x's having no chord. Circles thus produced may have n+1, n+3, ..., 3*n-1 points and up to n-1 chords. The circle may be rotated. a(n) is the count of unique patterns.

LINKS

Table of n, a(n) for n=0..9.

EXAMPLE

super-Catalan(3) = 11: (xx)xx, x(xx)x, xx(xx), (xx)(xx), (xxx)x, x(xxx),((xx)x)x, (x(xx))x, x((xx)x), x(x(xx)), xxxx. This sequence counts unique patterns up to rotation so  a(3) = 6: (xx)xx, (xx)(xx), (xxx)x, ((xx)x)x, x(x(xx)), xxxx.

CROSSREFS

Cf. A001003 super-Catalan numbers. A183758 similar but with reflections discounted. A183759 this series decomposed by number of chords in the circles.

Sequence in context: A264761 A253882 A327698 * A181490 A165325 A316765

Adjacent sequences:  A183754 A183755 A183756 * A183758 A183759 A183760

KEYWORD

nonn

AUTHOR

David Scambler, Jan 06 2011

STATUS

approved

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Last modified September 25 10:38 EDT 2020. Contains 337337 sequences. (Running on oeis4.)