Fuss–Catalan number

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In combinatorial mathematics and statistics, the Fuss–Catalan numbers are numbers of the form

A_m(p,r)\equiv\frac{r}{mp+r}\binom{mp+r}{m} = \frac{r}{m!}\prod_{i=1}^{m-1}(mp+r-i) = r\frac{\Gamma(mp+r)}{\Gamma(1+m)\Gamma(m(p-1)+r+1)}.

They are named after N. I. Fuss and Eugène Charles Catalan.

In some publications this equation is sometimes referred to as Two-parameter Fuss–Catalan numbers or Raney numbers. The implication is the single-parameter Fuss-Catalan numbers are when r=1.

Uses[edit]

The Fuss-Catalan represents the number of legal permutations or allowed ways of arranging a number of articles, that is restricted in some way. This means that they are related to the Binomial Coefficient. The key difference between Fuss-Catalan and the Binomial Coefficient is that there are no "illegal" arrangement permutations within Binomial Coefficient, but there are within Fuss-Catalan. An examples of legal and illegal permutations can be better demonstrated by a specific problem such as balanced brackets (see Dyck language).

A general problem is to count the number of balanced brackets (or legal permutations) that a string of 2m brackets forms. By legally arranged, the following rules apply:

  • For the sequence as a whole, the number of open brackets must equal the number of closed brackets
  • Working along the sequence, the difference between the number of open and closed brackets cannot be negative

As an numeric example how many combinations can 6 brackets be legally arranged? From the Binomial interpretation there are \tbinom {2m}{m} or numerically \tbinom 63 = 20 ways of arranging 3 open and 3 closed brackets. However, there are fewer legal combinations than these when all of the above restrictions apply. Evaluating these by hand, there are 5 legal combinations, namely: ()()(); (())(); ()(()); (()()); ((())). This corresponds to the Fuss-Catalan formula when p=2, r=1 which is the Catalan number formula \tfrac{1}{2m+1}\tbinom{2m}{m} or \tfrac{1}{4}\tbinom63=5. By simple subtraction, there are \tfrac{m}{m+1}\tbinom{2m}{m} or \tfrac34\tbinom63 =15 illegal combinations. To further illustrate the subtlety of the problem, if one were to persist with solving the problem just using the Binomial formula, it would be realised that the 2 rules imply that the sequence must start with an open bracket and finish with a closed bracket. This implies that there are \tbinom{2m-2}{m-1} or \tbinom42=6 combinations. This is inconsistent with the above answer of 5, and the missing combination is: ())((), which is illegal and would complete the binomial interpretation.

Whilst the above is a concrete example Catalan numbers, similar problems can be evaluated using Fuss-Catalan formula:

  • Computer Stack: ways of arranging and completing a computer stack of instructions, each time step 1 instruction is processed and p new instructions arrive randomly. If at the beginning of the sequence there are r instructions outstanding.
  • Betting: ways of losing all money when betting. A player has a total stake pot that allows them to make r bets, and plays a game of chance that pays p times the bet stake.
  • Tries: Calculating the number of order m tries on n nodes.[1]

Special Cases[edit]

A_0(p,r)=1;
A_1(p,r)=r;
A_{2}(p,1) = p.

If p=0, we recover the Binomial coefficients A_m(0,r){{=}}\binom{r}{m}

A_m(0,1) = 1,1;
A_m(0,2) = 1,2,1;
A_m(0,3) = 1,3,3,1;
A_m(0,4) = 1,4,6,4,1.

If p=1, Pascal's Triangle appears, read along diagonals:

A_m(1,1) = 1,1,1,1,1,1,1,1,1,1,\ldots;
A_m(1,2) = 1,2,3,4,5,6,7,8,9,10,\ldots;
A_m(1,3) = 1,3,6,10,15,21,28,35,45,55,\ldots;
A_m(1,4) = 1,4,10,20,35,56,84,120,165,220,\ldots;
A_m(1,5) = 1,5,15,35,70,126,210,330,495,715,\ldots;
A_m(1,6) = 1,6,21,56,126,252,462,792,1287,2002,\ldots.

Examples[edit]

For subindex m\ge 0 the numbers are:

Examples with p=2:

A_m(2,1) = 1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862,\ldots OEISA000108, known as the Catalan Numbers
A_m(2,2) = 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, \ldots = A_{m+1}(2,1)
A_m(2,3) = 1, 3, 9, 28, 90, 297, 1001, 3432, 11934, 41990, \ldots OEISA000245
A_m(2,4) = 1, 4, 14, 48, 165, 572, 2002, 7072, 25194, 90440,\ldots OEISA002057

Examples with p=3:

A_m(3,1) = 1, 1, 3, 12, 55, 273, 1428, 7752, 43263, 246675, \ldots OEISA001764
A_m(3,2) = 1, 2, 7, 30, 143, 728, 3876, 21318, 120175, 690690,\ldots OEISA006013
A_m(3,3) = 1, 3, 12, 55, 273, 1428, 7752, 43263, 246675, 1430715,\ldots = A_{m+1}(3,1)
A_m(3,4) = 1, 4, 18, 88, 455, 2448, 13566, 76912, 444015, 2601300,\ldots OEISA006629

Examples with p=4:

A_m(4,1) =  1, 1, 4, 22, 140, 969, 7084, 53820, 420732, 3362260,\ldots OEISA002293
A_m(4,2) =  1, 2, 9, 52, 340, 2394, 17710, 135720, 1068012, 8579560,\ldots OEISA069271
A_m(4,3) =  1, 3, 15, 91, 612, 4389, 32890, 254475, 2017356, 16301164, \ldots OEISA006632
A_m(4,4) =  1, 4, 22, 140, 969, 7084, 53820, 420732, 3362260, 27343888,\ldots = A_{m+1}(4,1)

Algebra[edit]

Recurrence[edit]

A_m(p,r)=A_m(p,r-1)+A_{m-1}(p,p+r-1) equation (1)

This means in particular that from

A_m(p,0)=0 equation (2)

and

A_0(p,r)=1 equation (3)

one can generate all other Fuss–Catalan numbers if p is an integer.

Riordan (see references) obtains a convolution type of recurrence:

A_m(p,s+r) = \sum_{k=0}^m A_k(p,r)A_{m-k}(p,s) equation(4)

Generating Function[edit]

Paraphrasing the Densities of the Raney distributions [2] paper, let the ordinary generating function with respect to the index m be defined as follows:

B_{p,r}(z):=\sum_{m=0}^\infty A_m(p,r)z^m equation (5).

Looking at equations (1) and (2), when r=1 it follows that

A_m(p,p)=A_{m+1}(p,1) equation (6).

Also note this result can be derived by similar substitutions into the other formulas representation, such as the Gamma ratio representation at the top of this article. Using (6) and substituting into (5) an equivalent representation expressed as a generating function can be formulated as

B_{p,1}(z) = 1+zB_{p,p}(z). equation (7).

Finally, extending this result by using Lambert's equivalence

B_{p,1}(z)^r=B_{p,r}(z) equation (8).

The following result can be derived for the ordinary generating function for all the Fuss-Catalan sequences.

B_{p,r}(z) = [1+zB_{p,r}(z)^{p/r}]^r.

Alternate representations[edit]

In some problems it is easier to use different formula configurations or variations. Below are a two examples using just the binomial function:

A_m(p,r)\equiv\frac{r}{mp+r}\binom{mp+r}{m} = \frac{r}{m(p-1)+r}\binom{mp+r-1}{m} = \frac{r}{m}\binom{mp+r-1}{m-1}

Note the last variant is problematic when m=0!! These variants can be converted into product, Gamma or Factorial representations too.

See also[edit]

References[edit]

  1. ^ Clark, David (1996). "Compact Tries". Compact Pat Trees (Thesis). p. 34. 
  2. ^ Mlotkowski, Wojciech; Penson, Karol A.; Zyczkowski, Karol (2013). "Densities of the Raney distributions". Docum. Mathm. 18: 1593–1596. 
  • Fuss, N. I. (1791). "Solutio quaestionis, quot modis polygonum n laterum in polygona m laterum, per diagonales resolvi queat". Nova Acta Academiae Sci. Petropolitanae 9: 243–251. 
  • Riordan, J. (1968). Combinatorial identities. Wiley. ISBN 978-0471722755. 
  • Bisch, Dietmar; Jones, Vaughan (1997). "Algebras associated to intermediate subfactors". Invent. Mathem. 128 (1): 89–157. doi:10.1007/s002220050137. 
  • Przytycki, Jozef H.; Sikora, Adam S. (2000). "Polygon dissections and Euler, Fuss, Kirkman , and Cayley Numbers". J. Combinat. Theory A 92: 68–76. doi:10.1006/jcta.1999.3042. 
  • Aval, Jean-Christophe (2008). "Multivariate Fuss-Catalan numbers". Discr. Math. 20 (308): 4660–4669. doi:10.1016/j.disc.2007.08.100. 
  • Alexeev, N.; Götze, F; Tikhomirov, A. (2010). "Asymptotic distribution of singular values of powers of random matrices". Lith. Math. J. 50 (2): 121–132. doi:10.1007/s10986-010-9074-4. 
  • Mlotkowski, Wojciech (2010). "Fuss-Catalan Numbers in noncommutative probability". Docum. Mathm. 15: 939–955. 
  • Penson, Karol A.; Zyczkowski, Karol (2011). "Product of Ginibre matrices: Fuss-Catalan and Raney distributions". Phys. Rev. E 83: 061118. Bibcode:2011PhRvE..83f1118P. doi:10.1103/PhysRevE.83.061118. 
  • Gordon, Ian G.; Griffeth, Stephen (2012). "Catalan numbers for complex reflection groups". Am. J. Math. 134 (6): 1491–1502. doi:10.1353/ajm.2012.0047.