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Catalan number : ウィキペディア英語版
Catalan number

In combinatorial mathematics, the Catalan numbers form a sequence of natural numbers that occur in various counting problems, often involving recursively-defined objects. They are named after the Belgian mathematician Eugène Charles Catalan (1814–1894).
Using zero-based numbering, the ''n''th Catalan number is given directly in terms of binomial coefficients by
:C_n = \frac = \frac = \prod\limits_^\frac \qquad\mboxn\ge 0.
The first Catalan numbers for ''n'' = 0, 1, 2, 3, … are
:1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, 742900, 2674440, 9694845, 35357670, 129644790, 477638700, 1767263190, 6564120420, 24466267020, 91482563640, 343059613650, 1289904147324, 4861946401452, … .
== Properties ==
An alternative expression for ''C''''n'' is
:C_n = - = \quad\textn\ge 0,
which is equivalent to the expression given above because \tbinom=\tfrac\tbinomn. This shows that ''C''''n'' is an integer, which is not immediately obvious from the first formula given. This expression forms the basis for a proof of the correctness of the formula.
The Catalan numbers satisfy the recurrence relation
:C_0 = 1 \quad \mbox \quad C_=\sum_^C_i\,C_\quad\textn\ge 0;
moreover,
:C_n= \frac 1 \sum_^n ^2.
This is because \tbinom = \sum_^n \tbinom^2, since choosing ''n'' numbers from a 2''n'' set of numbers can be uniquely divided into 2 parts: choosing ''i'' numbers out of the first ''n'' numbers and then choosing ''n''-''i'' numbers from the remaining ''n'' numbers.
They also satisfy:
:C_0 = 1 \quad \mbox \quad C_=\fracC_n,
which can be a more efficient way to calculate them.
Asymptotically, the Catalan numbers grow as
:C_n \sim \frac}
in the sense that the quotient of the ''n''th Catalan number and the expression on the right tends towards 1 as ''n'' → +∞. Some sources use just C_n \sim \frac\sqrt}. This means that the Catalan numbers are a solution of the Hausdorff moment problem on the interval (4 ) instead of (1 ). The orthogonal polynomials having the weight function \rho(x) on (4 ) are
:H_n(x)=\sum_^n(-x)^k.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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