The Catalan numbers on nonnegative integers 
are a set of numbers that arise in tree enumeration problems of the type, "In how many ways can a regular 
-gon be divided into 
triangles if different orientations are counted separately?" (Euler's polygon division problem). The solution is the Catalan number 
(Pólya 1956; Dörrie 1965; Honsberger 1973; Borwein and Bailey 2003, pp. 21-22), as graphically illustrated above (Dickau).
Catalan numbers are commonly denoted 
(Graham et al. 1994; Stanley 1999b, p. 219; Pemmaraju and Skiena 2003, p. 169; this work) or 
(Goulden and Jackson 1983, p. 111), and less commonly 
(van Lint and Wilson 1992, p. 136).
Catalan numbers are implemented in the Wolfram Language as CatalanNumber[n].
The first few Catalan numbers for 
, 2, ... are 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, ... (OEIS A000108).
Explicit formulas for 
include
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(1)
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(2)
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(3)
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(4)
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(5)
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(6)
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(7)
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where 
is a binomial coefficient, 
is a factorial, 
is a double factorial, 
is the gamma function, and 
is a hypergeometric function.
The Catalan numbers may be generalized to the complex plane, as illustrated above.
Sums giving 
include
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(8)
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(9)
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(10)
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(11)
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![sum_(k=0)^(|_n/2_|)[(n-2k+1)/(n-k+1)(n; n-k)]^2,](/images/equations/CatalanNumber/Inline51.svg) |
(12)
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where 
is the floor function, and a product for 
is given by
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(13)
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Sums involving 
include the generating function
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(14)
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(15)
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(OEIS A000108), exponential generating function
![e^(2x)[I_0(2x)-I_1(2x)]](/images/equations/CatalanNumber/Inline61.svg) |
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(16)
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(17)
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(OEIS A144186 and A144187), where 
is a modified Bessel function of the first kind, as well as
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(18)
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(19)
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The asymptotic form for the Catalan numbers is
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(20)
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(Vardi 1991, Graham et al. 1994).
The numbers of decimal digits in 
for 
, 1, ... are 1, 5, 57, 598, 6015, 60199, 602051, 6020590, ... (OEIS A114466). The digits converge to the digits in the decimal expansion of 
(OEIS A114493).
A recurrence relation for 
is obtained from
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(21)
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so
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(22)
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Segner's recurrence formula, given by Segner in 1758, gives the solution to Euler's polygon division problem
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(23)
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With 
, the above recurrence relation gives the Catalan number 
.
From the definition of the Catalan number, every prime divisor of 
is less than 
. On the other hand, 
for 
. Therefore, 
is the largest Catalan prime, making 
and 
the only Catalan primes. (Of course, much more than this can be said about the factorization of 
.)
The only odd Catalan numbers are those of the form 
. The first few are therefore 1, 5, 429, 9694845, 14544636039226909, ... (OEIS A038003).
The odd Catalan numbers 
end in 5 unless the base-5 expansion of 
uses only the digits 0, 1, 2, so it would be extremely rare for a long sequence of essentially random base-5 digits to contain only in 0, 1, and 2. In fact, the last digits of the odd Catalan numbers are 1, 5, 9, 5, 9, 5, 9, 7, 5, 5, 5, 5, 5, ... (OEIS A094389), so 5 is the last digit for all 
up to at least 
with the exception of 1, 3, 5, 7, and 8.
The Catalan numbers turn up in many other related types of problems. The Catalan number 
also gives the number of binary bracketings of 
letters (Catalan's problem), the solution to the ballot problem, the number of trivalent planted planar trees (Dickau; illustrated above), the number of states possible in an 
-flexagon, the number of different diagonals possible in a frieze pattern with 
rows, the number of Dyck paths with 
strokes, the number of ways of forming an 
-fold exponential, the number of rooted planar binary trees with 
internal nodes, the number of rooted plane bushes with 
graph edges, the number of extended binary trees with 
internal nodes, and the number of mountains which can be drawn with 
upstrokes and 
downstrokes, the number of noncrossing handshakes possible across a round table between 
pairs of people (Conway and Guy 1996)!
A generalization of the Catalan numbers is defined by
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(24)
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(25)
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for 
(Klarner 1970, Hilton and Pedersen 1991). The usual Catalan numbers 
are a special case with 
. 
gives the number of 
-ary trees with 
source-nodes, the number of ways of associating 
applications of a given 
-ary operator, the number of ways of dividing a convex polygon into 
disjoint 
-gons with nonintersecting polygon diagonals, and the number of p-good paths from (0, 
) to 
(Hilton and Pedersen 1991).
A further generalization is obtained as follows. Let 
be an integer 
, let 
with 
, and 
. Then define 
and let 
be the number of p-good paths from (1, 
) to 
(Hilton and Pedersen 1991). Formulas for 
include the generalized Jonah formula
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(26)
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and the explicit formula
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(27)
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A recurrence relation is given by
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(28)
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where 
, 
, 
, and 
(Hilton and Pedersen 1991).
