Narayana polynomials explained

Narayana polynomials are a class of polynomials whose coefficients are the Narayana numbers. The Narayana numbers and Narayana polynomials are named after the Canadian mathematician T. V. Narayana (1930–1987). They appear in several combinatorial problems.^{[1] [2] [3]}

Definitions

For a positive integer

n and for an integer k\geq 0 , the Narayana number N(n,k) is defined byN(n,k) = (1)/(n) {n \choose k}{n\choose k -1 }. The number N( 0 ,k) is defined as 1 for k =0 and as 0 for k\ne 0 .

For a nonnegative integer

n , the n -th Narayana polynomial N _{n(z)} is defined byN _{n(z)} = \sum ^{n} _{k =0} N(n,k)z ^{k.}

The associated Narayana polynomial

l N _{n(z)} is defined as the reciprocal polynomial of N _{n(z)} :l N ^{nN} _{n\left(\tfrac{ 1 }{z}\right)} .

Examples

The first few Narayana polynomials are

N _{0(z)=1} N _{1(z)=z} N ^{2+ z} _{2 (z) = z} N ^{3+3 z} _{3 (z) = z} ^{2+z} N ^{4+6 z} _{4 (z) = z} ^{3+6z} ^{2+z} N ^{5+10 z} _{5 (z) = z} ^{4+20z} ^{3+10z} ^{2+z}

Properties

A few of the properties of the Narayana polynomials and the associated Narayana polynomials are collected below. Further information on the properties of these polynomials are available in the references cited.

Alternative form of the Narayana polynomials

The Narayana polynomials can be expressed in the following alternative form:^[4]

N _{n(z)=} \sum ^{n} _{0} (1)/(n +1) {n +1 \choose k}{ 2 n - k \choose n}(z -1 ) ^{k}

Special values

N _{n(1)} is the n -th Catalan number C _{n = (1)/(n +1)} { 2 n \choose n} . The first few Catalan numbers are 1 , 1 , 2 , 5 , 14 , 42 , 132 , 429 , \ldots . .^[5] N _{n(2)} is the n -th large Schröder number. This is the number of plane trees having n edges with leaves colored by one of two colors. The first few Schröder numbers are 1 , 2 , 6 , 22 , 90 , 394 , 1806 , 8558 , \ldots . .^[5]

• For integers

n\ge 0 , let d _{n} denote the number of underdiagonal paths from ( 0 , 0 ) to (n,n) in a n x n grid having step set S = \{(k, 0 ) : k \in N ^{+\}} \cup \{( 0 , k) : k \in N ^{+\}} . Then d _{n} = l N( 4 ) .^[6]

Recurrence relations

• For

n \ge 3 , l N _{n(z)} satisfies the following nonlinear recurrence relation:^[6] l N _{n(z)} = ( 1+ z)N _{n-1} (z) + z \sum ^{n -2} _{k =1} l N _{k(z)lN} _{n-k-1} (z) .

• For

n\ge 3 , l N _{n(z)} satisfies the following second order linear recurrence relation:^[6] (n +1 ) l N _{n(z)} = ( 2 n -1 )( 1+ z) l N _{n-1} (z) - (n -2 )(z -1 ) ^{2 l N} _{n -2} (z) with l N _{1(z)=1} and l N _{2(z)=1+z} .

Generating function

The ordinary generating function the Narayana polynomials is given by

\sum ^{inf ty} _{n =0} N ^{n} _{n(z)t} = (1+ t - t z - \sqrt{ 1-2 ( 1+ z) t + ( 1- z) ^{2} t ^{2})/() }.

Integral representation

The

n -th degree Legendre polynomial P _{n(x)} is given byP _{n(x)} = 2 ^{-n} \sum ^{\left\lfloor (n)/(2) \right\rfloor} _{k =0} ( -1 ) ^{k} {n - k \choose k}{ 2 n -2 k \choose n - k}x ^{n-2k} Then, for n > 0, the Narayana polynomial N _{n(z)} can be expressed in the following form:N ^{n +1} _{n(z) = (z -1 )} \int ^{(z)/(z -1)} _{0} P _{n(2x-1)dx} .

See also

• Catalan number
• Schröder number

Notes and References

1. D. G. Rogers . Rhyming schemes: Crossings and coverings . Discrete Mathematics . 1981 . 33 . 67–77 . 10.1016/0012-365X(81)90259-4 . 2 December 2023.
2. Book: R.P. Stanley . Enumerative Combinatorics, Vol. 2 . 1999 . Cambridge University Press.
3. Rodica Simian and Daniel Ullman . On the structure of the lattice of noncrossing partitions . Discrete Mathematics . 1991 . 98 . 3 . 193–206 . 10.1016/0012-365X(91)90376-D . 2 December 2023.
4. Ricky X. F. Chen and Christian M. Reidys . Narayana polynomials and some generalizations . 2014 . math.CO . 1411.2530 .
5. Toufik Mansour, Yidong Sun . Identities involving Narayana polynomials and Catalan numbers . 2008 . math.CO . 0805.1274 .
6. Curtis Coker . Enumerating a class oflattice paths . Discrete Mathematics . 2003 . 271 . 1–3 . 13–28 . 10.1016/S0012-365X(03)00037-2 . 1 December 2023.