OFFSET
0,3
COMMENTS
Definition: Given a primitive k-th root of unity w, a binary operation a*b=a+wb, and sufficiently general fixed complex numbers x_0, ..., x_n, the k-modular Catalan numbers C_{n,k} enumerate parenthesizations of x_0*x_1*...*x_n that give distinct values.
Theorem: C_{n,k} enumerates the following objects:
(1) binary trees with n internal nodes avoiding a certain subtree (i.e., comb_k^{+1}),
(2) plane trees with n+1 nodes whose non-root nodes have degree less than k,
(3) Dyck paths of length 2n avoiding a down-step followed immediately by k consecutive up-steps,
(4) partitions with n nonnegative parts bounded by the staircase partition (n-1,n-2,...,1,0) such that each positive number appears fewer than k times,
(5) standard 2-by-n Young tableaux whose top row avoids contiguous labels of the form i,j+1,j+2,...,j+k for all i<j, and
(6) permutations of {1,2,...,n} avoiding 1-3-2 and 23...(k+1)1.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..200
Nickolas Hein and Jia Huang, Modular Catalan Numbers, arXiv:1508.01688 [math.CO], 2015.
Nickolas Hein and Jia Huang, Modular Catalan Numbers, European Journal of Combinatorics 61 (2017), 197-218.
FORMULA
sum( 1<=l<=n, (l/n)sum( m_1+...+m_k=n and m_2+2m_3+...+(k-1)m_k=n-l, MC(n;m_1,...,m_k) ) ), where MC(n;m_1,...,m_k) is the multinomial coefficient associated to the multiset (m_1,...,m_k).
G.f.: 1/(1-x*G(x)) where G(x) is g.f. of A036766. - Andrew Howroyd, Dec 04 2017
Recurrence: 3*n*(3*n - 2)*(3*n - 1)*(1309*n^5 - 14388*n^4 + 60934*n^3 - 124236*n^2 + 121825*n - 45948)*a(n) = (299761*n^8 - 3779182*n^7 + 19492177*n^6 - 53378731*n^5 + 84116656*n^4 - 77081911*n^3 + 39268230*n^2 - 9775512*n + 829440)*a(n-1) - 5*(119119*n^8 - 1601215*n^7 + 8920729*n^6 - 26755339*n^5 + 46820344*n^4 - 48217102*n^3 + 27785664*n^2 - 7773768*n + 712800)*a(n-2) - 25*(n-3)*(1309*n^7 - 11770*n^6 + 38824*n^5 - 62344*n^4 + 74887*n^3 - 107794*n^2 + 101952*n - 33120)*a(n-3) - 125*(n-4)*(n-3)*(1309*n^6 - 10461*n^5 + 28528*n^4 - 30261*n^3 + 7999*n^2 + 3390*n - 1080)*a(n-4) - 625*(n-5)*(n-4)*(n-3)*(1309*n^5 - 7843*n^4 + 16472*n^3 - 14672*n^2 + 5148*n - 504)*a(n-5). - Vaclav Kotesovec, Dec 05 2017
EXAMPLE
The Catalan number C_6=132 counts the parenthesizations of x_1*...*x_7 where * is arbitrary. Defining * and w as above and writing x_i compactly as xi, we have x1*(x2*(x3*(x4*(x5*(x6*(x7)))))) = x1+wx2+w^2x3+w^3x4+w^4x5+x6+wx7 = x1*(x2*(x3*(x4*(x5*(x6)))))*(x7). For n=6 and k=5, these are the only parenthesizations that give the same value for x1*...*x7, so C_{6,5}=132-1=131.
MATHEMATICA
terms = 30; col[k_] := Module[{G}, G = InverseSeries[x*(1 - x)/(1 - x^k) + O[x]^terms, x]; CoefficientList[1/(1 - G), x]];
col[5] (* Jean-François Alcover, Dec 05 2017, after Andrew Howroyd *)
PROG
(PARI) Vec(1/(1-serreverse(x*(1-x)/(1-x^5) + O(x*x^25)))) \\ Andrew Howroyd, Dec 04 2017
(Sage)
def C(k):
print(1)
for n in range(1, 51):
f = ((1-x^k)/(1-x))^n # ((x+1)^2-x^2*(x/(x+1))^(k-2))^n
f = f.simplify_full()
C = 0
for i in range(n):
C = C + (n-i)*f.coefficient(x, i)/n
print(C)
time C(5)
(Sage)
print(0, 1)
nbound=100
for nMinus1 in range(nbound):
n=nMinus1+1
Cn=0
for lMinus1 in range(n):
l=lMinus1+1
for m5 in range(1+(n-l)/4):
for m4 in range(1+(n-l-4*m5)/3):
for m3 in range(1+(n-l-4*m5-3*m4)/2):
m2=n-l-4*m5-3*m4-2*m3
m1=n-m2-m3-m4-m5
Cn=Cn+l*sage.rings.arith.multinomial(m1, m2, m3, m4, m5)/n
print(Cn)
CROSSREFS
KEYWORD
nonn
AUTHOR
Nickolas Hein, Aug 25 2015
STATUS
approved