Article

General Formulas for the Central and Non-Central Moments of the Multinomial Distribution

Department of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
Stats 2021, 4(1), 18-27; https://doi.org/10.3390/stats4010002
Received: 10 November 2020 / Revised: 31 December 2020 / Accepted: 5 January 2021 / Published: 6 January 2021
(This article belongs to the Section Multivariate Analysis)

Abstract

We present the first general formulas for the central and non-central moments of the multinomial distribution, using a combinatorial argument and the factorial moments previously obtained in Mosimann (1962). We use the formulas to give explicit expressions for all the non-central moments up to order 8 and all the central moments up to order 4. These results expand significantly on those in Newcomer (2008) and Newcomer et al. (2008), where the non-central moments were calculated up to order 4.
Keywords: multinomial distribution; higher moments; central moments; non-central moments multinomial distribution; higher moments; central moments; non-central moments

1. The Multinomial Distribution

For any dN, the d-dimensional (unit) simplex is defined by S:=x[0,1]d:i=1dxi1, and the probability mass function kPk,m(x) for ξ:=(ξ1,ξ2,,ξd)Multinomial(m,x) is defined by
Pk,m(x):=m!(mi=1dki)!i=1dki!·(1i=1dxi)mi=1dkii=1dxiki,kN0dmS,
where mN and xS. If xd+1:=1i=1dxi, then (1) is just a reparametrization of (ξ,1i=1dξi)Multinomial(m,(x,xd+1)) where i=1d+1xi=1. In this paper, our main goal is to give general formulas for the non-central and central moments of (1), namely
Ei=1dξipiandEi=1d(ξiE[ξi])pi,p1,p2,,pdN0.
We obtain the formulas using a combinatorial argument and the general expression for the factorial moments found in Mosimann (1962) [1], which we register in the lemma below.
Lemma 1
(Factorial moments). Let ξMultinomial(m,x). Then, for all k1,k2,,kdN0,
Ei=1dξi(ki)=m(i=1dki)i=1dxki,
where m(k):=m(m1)(mk+1) denotes the k-th order falling factorial of m.
The formulas that we develop for the expectations in (2) will be used to compute explicitly all the non-central moments up to order 8 and all the central moments up to order 4, which expands on the third and fourth order non-central moments that were previously calculated in (Newcomer (2008) [2], Appendix A.1). We should also mention that explicit formulas for several lower-order (mixed) cumulants were presented in Wishart (1949) [3] (see also Johnson et al. (1997) [4], page 37), but not for the moments.

2. Motivation

To the best of our knowledge, general formulas for the central and non-central moments of the multinomial distribution have never been derived in the literature. The central moments can arise naturally, for example, when studying asymptotic properties, via Taylor expansions of statistical estimators involving the multinomial distribution. For a given sequence of i.i.d. observations X1,X2,,Xn, two examples of such estimators are the Bernstein estimator for the cumulative distribution function
Fn,m(x):=kN0dmS1ni=1n𝟙(,km](Xi)Pk,m(x),xS,m,nN,
and the Bernstein estimator for the density function (also called smoothed histogram)
f^n,m(x):=kN0d(m1)Smdni=1n𝟙(km,k+1m](Xi)Pk,m1(x),xS,m,nN,
over the d-dimensional simplex. Some of their asymptotic properties were investigated by Vitale (1975), Stadtmüller (1986), Tenbusch (1997), Petrone (1999), Ghosal (2001), Petrone and Wasserman (2002), Babu et al. (2002), Kakizawa (2004), Bouezmarni and Rolin (2007), Bouezmarni et al. (2007), Leblanc (2009, 2010, 2012), Curtis and Ghosh (2011), Igarashi and Kakizawa (2014), Turnbull and Ghosh (2014), Lu (2015), Guan (2016, 2017) and Belalia et al. (2017, 2019) [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30] when d=1, by Tenbusch (1994) [31] when d=2, and by Ouimet (2020) [32,33] for all d1, using a local limit theorem from Ouimet (2020) [34] for the multinomial distribution (see also Arenbaev (1976) [35]). The estimator (5) is a discrete analogue of the Dirichlet kernel estimator introduced by Aitchison and Lauder (1985) [36] and studied theoretically in Brown and Chen (1999), Chen (1999, 2000) and Bouezmarni and Rolin (2003) [37,38,39,40] when d=1 (among others), and in Ouimet (2020) [41] for all d1.

3. Results

First, we give a general formula of the non-central moments of the multinomial distribution in (1).
Theorem 1
(Non-central moments). Let ξMultinomial(m,x). For all p1,p2,,pdN0,
Ei=1dξipi=k1=0p1kd=0pdm(i=1dki)i=1dpikixiki,
where pk denotes a Stirling number of the second kind (i.e., the number of ways to partition a set of p objects into k non-empty subsets).
Proof. 
We have the following well-known relation between the power pN0 of a number xR and the falling factorials of x:
xp=k=0ppkx(k).
See, e.g., (Graham et al. (1994) [42], page 262). Apply this relation to every ξipi and use the linearity of the expectation to get
Ei=1dξipi=k1=0p1kd=0pdp1k1pdkdEi=1dξi(ki).
The conclusion follows from Lemma 1.    □
We deduce a general formula for the central moments of the multinomial distribution.
Theorem 2
(Central moments). Let ξMultinomial(m,x). For all p1,p2,,pdN0,
Ei=1d(ξiE[ξi])pi=1=0p1d=0pdk1=01kd=0dm(i=1dki)(m)i=1d(pii)i=1dpiiikixipii+ki,
where p denotes the binomial coefficient p!!(p)!.
Proof. 
By applying the binomial formula to each factor (ξiE[ξi])pi and using the fact that E[ξi]=mxi for all i{1,2,,d}, note that
Ei=1d(ξiE[ξi])pi=1=0p1d=0pdEi=1dξii·i=1dpii(mxi)pii.
The conclusion follows from Theorem 1.    □

4. Numerical Implementation

The formulas in Theorems 1 and  2 can be implemented in Mathematica as follows:
  NonCentral[m_, x_, p_, d_] :=
    Sum[FactorialPower[m, Sum[k[i], {i,1,d}]] *
    Product[StirlingS2[p[[i]], k[i]] * x[[i]] ^ k[i], {i,1,d}], ##] & @@
    ({k[#], 0, p[[#]]} & /@ Range[d]);
  Central[m_, x_, p_, d_] :=
    Sum[Sum[FactorialPower[m, Sum[k[i], {i,1,d}]] *
    (-m) ^ Sum[p[[i]] - ell[i], {i,1,d}] *
    Product[Binomial[p[[i]], ell[i]] * StirlingS2[ell[i], k[i]] *
    x[[i]] ^ (p[[i]] - ell[i] + k[i]), {i,1,d}], ##] & @@
    ({k[#], 0, ell[#]} & /@ Range[d]), ##] & @@
    ({ell[#], 0, p[[#]]} & /@ Range[d]);

5. Explicit Formulas

In Newcomer (2008) [2], explicit expressions for the non-central moments of order 3 and 4 were obtained for the multinomial distribution, see also Newcomer et al. (2008) and Ouimet (2020) [43,44]. To expand on those results, we use the formula from Theorem 1 in the two subsections below to calculate (explicitly) all the non-central moments up to order 8 and all the central moments up to order 4.
Here is a table of the Stirling numbers of the second kind that we will use in our calculations:
00=1,10=0,11=1,20=0,21=1,22=1,30=0,31=1,32=3,33=1,40=0,41=1,42=7,43=6,44=1,50=0,51=1,52=15,53=25,54=10,55=1,60=0,61=1,62=31,63=90,64=65,65=15,66=1,70=0,71=1,72=63,73=301,74=350,75=140,76=21,77=1,80=0,81=1,82=127,83=966,84=1701,85=1050,86=266,87=28,88=1.

5.1. Computation of the Non-Central Moments Up to Order 8

By applying the general expression in Theorem 1 and by removing the Stirling numbers piki that are equal to 0, we get the following results directly.
Order 1: For any j1{1,2,,d},
E[ξj1]=xj1m.
Order 2: For any distinct j1,j2{1,2,,d},
E[ξj12]=xj1[m+m(2)xj1],
E[ξj1ξj2]=xj1xj2m(2).
Order 3: For any distinct j1,j2,j3{1,2,,d},
E[ξj13]=xj1[m+3m(2)xj1+m(3)xj12],
E[ξj12ξj2]=xj1xj2[m(2)+m(3)xj1],
E[ξj1ξj2ξj3]=xj1xj2xj3m(3).
Order 4: For any distinct j1,j2,j3,j4{1,2,,d},
E[ξj14]=xj1[m+7m(2)xj1+6m(3)xj12+m(4)xj13],
E[ξj13ξj2]=xj1xj2[m(2)+3m(3)xj1+m(4)xj12],
E[ξj12ξj22]=xj1xj2[m(2)+m(3)(xj1+xj2)+m(4)xj1xj2],
E[ξj12ξj2ξj3]=xj1xj2xj3]m(3)+m(4)xj1],
E[ξj1ξj2ξj3ξj4]=xj1xj2xj3xj4m(4).
Order 5: For any distinct j1,j2,j3,j4,j5{1,2,,d},
E[ξj15]=xj1[m+15m(2)xj1+25m(3)xj12+10m(4)xj13+m(5)xj14],
E[ξj14ξj2]=xj1xj2[m(2)+7m(3)xj1+6m(4)xj12+m(5)xj13],
E[ξj13ξj22]=xj1xj2[m(2)+m(3)(3xj1+xj2)+m(4)(xj12+3xj1xj2)+m(5)xj12xj2],
E[ξj13ξj2ξj3]=xj1xj2xj3[m(3)+3m(4)xj1+m(5)xj12],
E[ξj12ξj22ξj3]=xj1xj2xj3[m(3)+m(4)(xj1+xj2)+m(5)xj1xj2],
E[ξj12ξj2ξj3ξj4]=xj1xj2xj3xj4[m(4)+m(5)xj1],
E[ξj1ξj2ξj3ξj4ξj5]=xj1xj2xj3xj4xj5m(5).
Order 6: For any distinct j1,j2,j3,j4,j5,j6{1,2,,d},
E[ξj16]=xj1[m+31m(2)xj1+90m(3)xj12+65m(4)xj13+15m(5)xj14+m(6)xj15],
E[ξj15ξj2]=xj1xj2[m(2)+15m(3)xj1+25m(4)xj12+10m(5)xj13+m(6)xj14],
E[ξj14ξj22]=xj1xj2m(2)+m(3)(7xj1+xj2)+m(4)(6xj12+7xj1xj2)+m(5)(xj13+6xj12xj2)+m(6)xj13xj2,
E[ξj14ξj2ξj3]=xj1xj2xj3[m(3)+7m(4)xj1+6m(5)xj12+m(6)xj13],
E[ξj13ξj23]=xj1xj2m(2)+m(3)(3xj1+3xj2)+m(4)(xj12+9xj1xj2+xj22)+m(5)(3xj12xj2+3xj1xj22)+m(6)xj12xj22,
E[ξj13ξj22ξj3]=xj1xj2xj3[m(3)+m(4)(3xj1+xj2)+m(5)(xj12+3xj1xj2)+m(6)xj12xj2],
E[ξj13ξj2ξj3ξj4]=xj1xj2xj3xj4[m(4)+3m(5)xj1+m(6)xj12],
E[ξj12ξj22ξj32]=xj1xj2xj3m(3)+m(4)(xj1+xj2+xj3)+m(5)(xj1xj2+xj1xj3+xj2xj3)+m(6)xj1xj2xj3,
E[ξj12ξj22ξj3ξj4]=xj1xj2xj3xj4[m(4)+m(5)(xj1+xj2)+m(6)xj1xj2],
E[ξj12ξj2ξj3ξj4ξj5]=xj1xj2xj3xj4xj5[m(5)+m(6)xj1],
E[ξj1ξj2ξj3ξj4ξj5ξj6]=xj1xj2xj3xj4xj5xj6m(6).
Order 7: For any distinct j1,j2,j3,j4,j5,j6,j7{1,2,,d},
E[ξj17]=xj1m+63m(2)xj1+301m(3)xj12+350m(4)xj13+140m(5)xj14+21m(6)xj15+m(7)xj16,
E[ξj16ξj2]=xj1xj2m(2)+31m(3)xj1+90m(4)xj12+65m(5)xj13+15m(6)xj14+m(7)xj15,
E[ξj15ξj22]=xj1xj2m(2)+m(3)(15xj1+xj2)+m(4)(25xj12+15xj1xj2)+m(5)(10xj13+25xj12xj2)+m(6)(xj14+10xj13xj2)+m(7)xj14xj2,
E[ξj15ξj2ξj3]=xj1xj2xj3[m(3)+15m(4)xj1+25m(5)xj12+10m(6)xj13+m(7)xj14],
E[ξj14ξj23]=xj1xj2m(2)+m(3)(7xj1+3xj2)+m(4)(6xj12+21xj1xj2+xj22)+m(5)(xj13+18xj12xj2+7xj1xj22)+m(6)(3xj13xj2+6xj12xj22)+m(7)xj13xj22,
E[ξj14ξj22ξj3]=xj1xj2xj3m(3)+m(4)(7xj1+xj2)+m(5)(6xj12+7xj1xj2)+m(6)(xj13+6xj12xj2)+m(7)xj13xj2,
E[ξj14ξj2ξj3ξj4]=xj1xj2xj3xj4[m(4)+7m(5)xj1+6m(6)xj12+m(7)xj13],
E[ξj13ξj23ξj3]=xj1xj2xj3m(3)+m(4)(3xj1+3xj2)+m(5)(xj12+9xj1xj2+xj22)+m(6)(3xj12xj2+3xj1xj22)+m(7)xj12xj22,
E[ξj13ξj22ξj32]=xj1xj2xj3m(3)+m(4)(3xj1+xj2+xj3)+m(5)(xj12+3xj1xj2+3xj1xj3+xj2xj3)+m(6)(xj12xj2+xj12xj3+3xj1xj2xj3)+m(7)xj12xj2xj3,
E[ξj13ξj22ξj3ξj4]=xj1xj2xj3xj4m(4)+m(5)(3xj1+xj2)+m(6)(xj12+3xj1xj2)+m(7)xj12xj2,
E[ξj13ξj2ξj3ξj4ξj5]=xj1xj2xj3xj4xj5[m(5)+3m(6)xj1+m(7)xj12],
E[ξj12ξj22ξj32ξj4]=xj1xj2xj3xj4m(4)+m(5)(xj1+xj2+xj3)+m(6)(xj1xj2+xj1xj3+xj2xj3)+m(7)xj1xj2xj3,
E[ξj12ξj22ξj3ξj4ξj5]=xj1xj2xj3xj4xj5m(5)+m(6)(xj1+xj2)+m(7)xj1xj2,
E[ξj12ξj2ξj3ξj4ξj5ξj6]=xj1xj2xj3xj4xj5xj6[m(6)+m(7)xj1],
E[ξj1ξj2ξj3ξj4ξj5ξj6ξj7]=xj1xj2xj3xj4xj5xj6xj7m(7).
Order 8: For any distinct j1,j2,j3,j4,j5,j6,j7,j8{1,2,,d},
E[ξj18]=xj1m+127m(2)xj1+966m(3)xj22+1701m(4)xj13+1050m(5)xj14+266m(6)xj15+28m(7)xj16+m(8)xj17,
E[ξj17ξj2]=xj1xj2m+63m(3)xj1+301m(4)xj12+350m(5)xj13+140m(6)xj14+21m(7)xj15+m(8)xj16,
E[ξj16ξj22]=xj1xj2m(2)+m(3)(31xj1+xj2)+m(4)(90xj12+31xj1xj2)+m(5)(65xj13+90xj12xj2)+m(6)(15xj14+65xj13xj2)+m(7)(xj15+15xj14xj2)+m(8)xj15xj2,
E[ξj16ξj2ξj3]=xj1xj2xj3m(3)+31m(4)xj1+90m(5)xj12+65m(6)xj13+15m(7)xj14+m(8)xj15,
E[ξj15ξj23]=xj1xj2m(2)+m(3)(15xj1+3xj2)+m(4)(25xj12+45xj1xj2+xj22)+m(5)(10xj13+75xj12xj2+15xj1xj22)+m(6)(xj44+30xj13xj2+25xj12xj22)+m(7)(3xj14xj2+10xj13xj22)+m(8)xj14xj22,
E[ξj15ξj22ξj3]=xj1xj2xj3m(3)+m(4)(15xj1+xj2)+m(5)(25xj12+15xj1xj2)+m(6)(10xj13+25xj12xj2)+m(7)(xj14+10xj13xj2)+m(8)xj14xj2,
E[ξj15ξj2ξj3ξj4]=xj1xj2xj3xj4[m(4)+15m(5)xj1+25m(6)xj12+10m(7)xj13+m(8)xj14],
E[ξj14ξj24]=xj1xj2m(2)+m(3)(7xj1+7xj2)+m(4)(6xj12+49xj1xj2+6xj22)+m(5)(xj13+42xj12xj2+42xj1xj22+xj23)+m(6)(7xj13xj2+36xj12xj22+7xj1xj23)+m(7)(6xj13xj22+6xj12xj23)+m(8)xj13xj23,
E[ξj14ξj23ξj3]=xj1xj2xj3m(3)+m(4)(7xj1+3xj2)+m(5)(6xj12+21xj1xj2+xj22)+m(6)(xj13+18xj12xj2+7xj1xj22)+m(7)(3xj13xj2+6xj12xj22)+m(8)xj13xj22,
E[ξj14ξj22ξj32]=xj1xj2xj3m(3)+m(4)(7xj1+xj2+xj3)+m(5)(6xj12+7xj1xj2+7xj1xj3+xj2xj3)+m(6)(xj13+6xj12xj2+6xj12xj3+7xj1xj2xj3)+m(7)(xj13xj2+xj13xj3+6xj12xj2xj3)+m(8)xj13xj2xj3,
E[ξj14ξj22ξj3ξj4]=xj1xj2xj3xj4m(4)+m(5)(7xj1+xj2)+m(6)(6xj12+7xj1xj2)+m(7)(xj13+6xj12xj2)+m(8)xj13xj2,
E[ξj14ξj2ξj3ξj4ξj5]=xj1xj2xj3xj4xj5[m(5)+7m(6)xj1+6m(7)xj12+m(8)xj13],
E[ξj13ξj23ξj32]=xj1xj2xj3m(3)+m(4)(3xj1+3xj2+xj3)+m(5)(xj12+xj22+3xj1xj3+3xj2xj3+9xj1xj2)+m(6)(xj12xj3+xj22xj3+3xj12xj2+3xj1xj22+9xj1xj2xj3)+m(7)(xj12xj22+3xj12xj2xj3+3xj1xj22xj3)+m(8)xj12xj22xj3,
E[ξj13ξj23ξj3ξj4]=xj1xj2xj3xj4m(4)+m(5)(3xj1+3xj2)+m(6)(xj12+9xj1xj2+xj22)+m(7)(3xj12xj2+3xj1xj22)+m(8)xj12xj22,
E[ξj13ξj22ξj32ξj4]=xj1xj2xj3xj4m(4)+m(5)(3xj1+xj2+xj3)+m(6)(3xj1xj2+3xj1xj3+xj2xj3)+m(7)(xj12xj2+xj12xj3+3xj1xj2xj3)+m(8)xj12xj2xj3,
E[ξj13ξj22ξj3ξj4ξj5]=xj1xj2xj3xj4xj5m(5)+m(6)(3xj1+xj2)+m(7)(xj12+3xj1xj2)+m(8)xj12xj2,
E[ξj13ξj2ξj3ξj4ξj5ξj6]=xj1xj2xj3xj4xj5xj6[m(6)+3m(7)xj1+m(8)xj12],
E[ξj12ξj22ξj32ξj42]=xj1xj2xj3xj4m(4)+m(5)(xj1+xj2+xj3+xj4)+m(6)(xj1xj2+xj1xj3+xj1xj4+xj2xj3+xj2xj4+xj3xj4)+m(7)(xj1xj2xj3+xj1xj2xj4+xj1xj3xj4+xj2xj3xj4)+m(8)xj1xj2xj3xj4,
E[ξj12ξj22ξj32ξj4ξj5]=xj1xj2xj3xj4xj5m(5)+m(6)(xj1+xj2+xj3)+m(7)(xj1xj2+xj1xj3+xj2xj3)+m(8)xj1xj2xj3,
E[ξj12ξj22ξj3ξj4ξj5ξj6]=xj1xj2xj3xj4xj5xj6[m(6)+m(7)(xj1+xj2)+m(8)xj1xj2],
E[ξj12ξj2ξj3ξj4ξj5ξj6ξj7]=xj1xj2xj3xj4xj5xj6xj7[m(7)+m(8)xj1],
E[ξj1ξj2ξj3ξj4ξj5ξj6ξj7ξj8]=xj1xj2xj3xj4xj5xj6xj7xj8m(8).

5.2. Computation of the Central Moments Up to Order 4

With the results of the previous subsection and some algebraic manipulations (or the formula in Theorem 2), we can now calculate the central moments explicitly. We could calculate them up to order 8, but it would be very tedious. Instead, we write them up to order 4 for the sake of brevity. The simplifications we make to obtain the boxed expressions below are done with Mathematica.
Order 2: For any distinct j1,j2{1,2,,d},
E[(ξj1E[ξj1])2]=E[ξj12](E[ξj1])2=xj1[m+m(2)xj1]m2xj12=mxj1(1xj1)
E[(ξj1E[ξj1])(ξj2E[ξj2])]=E[ξj1ξj2]E[ξj1]E[ξj2]=m(2)xj1xj2mxj1mxj2=mxj1xj2.
Order 3: For any distinct j1,j2,j3{1,2,,d},
E[(ξj1E[ξj1])3]=E[ξj13]3E[ξj12]E[ξj1]+2(E[ξj1])3=xj1[m+3m(2)xj1+m(3)xj12]3xj1[m+m(2)xj1]mxj1+2m3xj13=mxj1(xj11)(2xj11)
E[(ξj1E[ξj1])2(ξj2E[ξj2])]=E[ξj12ξj2]E[ξj12]E[ξj2]2E[ξj1ξj2]E[ξj1]+2(E[ξj1])2E[ξj2]=xj1xj2[m(2)+m(3)xj1]xj1m+m(2)xj1mxj22m(2)xj1xj2mxj1+2m2xj12mxj2=mxj1xj2(2xj11)
E[(ξj1E[ξj1])(ξj2E[ξj2])(ξj3E[ξj3])]=E[ξj1ξj2ξj3]E[ξj1ξj2]E[ξj3]E[ξj1ξj3]E[ξj2]E[ξj2ξj3]E[ξj1]+2E[ξj1]E[ξj2]E[ξj3]=m(3)xj1xj2xj3m(2)xj1xj2mxj3m(2)xj1xj3mxj2m(2)xj2xj3mxj1+2m3xj1xj2xj3=2mxj1xj2xj3.
E[(ξj1E[ξj1])4]=E[ξj14]4E[ξj13]E[ξj1]+6E[ξj12](E[ξj1])23(E[ξj1])4=xj1[m+7m(2)xj1+6m(3)xj12+m(4)xj13]4xj1[m+3m(2)xj1+m(3)xj12]mxj1+6xj1[m+m(2)xj1](mxj1)23m4xj14=3m2xj12(xj11)2+mxj1(1xj1)(6xj126xj1+1)
Order 4: For any distinct j1,j2,j3,j4{1,2,,d},
E[(ξj1E[ξj1])3(ξj2E[ξj2])]=E[ξj13ξj2]E[ξj13]E[ξj2]3E[ξj12ξj2]E[ξj1]+3E[ξj12]E[ξj1]E[ξj2]+3E[ξj1ξj2](E[ξj1])23(E[ξj1])3E[ξj2]=xj1xj2[m(2)+3m(3)xj1+m(4)xj12]xj1[m+3m(2)xj1+m(3)xj12]mxj23xj1xj2[m(2)+m(3)xj1]mxj1+3xj1[m+m(2)xj1]mxj1mxj2+3m(2)xj1xj2m2xj123m3xj13mxj2=mxj1xj2(3(m2)xj1(xj11)1)
E[(ξj1E[ξj1])2(ξj2E[ξj2])2]=E[ξj12ξj22]2E[ξj12ξj2]E[ξj2]2E[ξj1ξj22]E[ξj1]+E[ξj12](E[ξj2])2+E[ξj22](E[ξj1])2+4E[ξj1ξj2]E[ξj1]E[ξj2]3(E[ξj1])2(E[ξj2])2=xj1xj2[m(2)+m(3)(xj1+xj2)+m(4)xj1xj2]2xj1xj2[m(2)+m(3)xj1]mxj22xj1xj2[m(2)+m(3)xj2]mxj1+xj1[m+m(2)xj1]m2xj22+xj2[m+m(2)xj2]m2xj12+4m(2)xj1xj2mxj1mxj23m2xj12m2xj22=m(m2)xj1xj2(3xj1xj2(xj1+xj2)+1)+mxj1xj2
E[(ξj1E[ξj1])2(ξj2E[ξj2])(ξj3E[ξj3])]=E[ξj12ξj2ξj3]E[ξj12ξj2]E[ξj3]E[ξj12ξj3]E[ξj2]2E[ξj1ξj2ξj3]E[ξj1]+E[ξj12]E[ξj2]E[ξj3]+2E[ξj1ξj2]E[ξj1]E[ξj3]+2E[ξj1ξj3]E[ξj1]E[ξj2]+E[ξj2ξj3](E[ξj1])23(E[ξj1])2E[ξj2]E[ξj3]=xj1xj2xj3[m(3)+m(4)xj1]xj1xj2[m(2)+m(3)xj1]mxj3xj1xj3[m(2)+m(3)xj1]mxj22m(3)xj1xj2xj3mxj1+xj1[m+m(2)xj1]mxj2mxj3+2m(2)xj1xj2mxj1mxj3+2m(2)xj1xj3mxj1mxj2+m(2)xj2xj3m2xj123m2xj12mxj2mxj3=m(m2)xj1xj2xj3(3xj11)
E[(ξj1E[ξj1])(ξj2E[ξj2])(ξj3E[ξj3])(ξj4E[ξj4])]=E[ξj1ξj2ξj3ξj4]E[ξj1ξj2ξj3]E[ξj4]E[ξj1ξj2ξj4]E[ξj3]E[ξj1ξj3ξj4]E[ξj2]E[ξj2ξj3ξj4]E[ξj1]+E[ξj1ξj2]E[ξj3]E[ξj4]+E[ξj1ξj3]E[ξj2]E[ξj4]+E[ξj1ξj4]E[ξj2]E[ξj3]+E[ξj2ξj3]E[ξj1]E[ξj4]+E[ξj2ξj4]E[ξj1]E[ξj3]+E[ξj3ξj4]E[ξj1]E[ξj2]3E[ξj1]E[ξj2]E[ξj3]E[ξj4]=m(4)xj1xj2xj3xj4m(3)xj1xj2xj3mxj4m(3)xj1xj2xj4mxj3m(3)xj1xj3xj4mxj2m(3)xj2xj3xj4mxj1+m(2)xj1xj2mxj3mxj4+m(2)xj1xj3mxj2mxj4+m(2)xj1xj4mxj2mxj3+m(2)xj2xj3mxj1mxj4+m(2)xj2xj4mxj1mxj3+m(2)xj3xj4mxj1mxj23m4xj1xj2xj3xj4=3m(m2)xj1xj2xj3xj4.

6. Conclusions

In this short paper, we found general formulas for the central and non-central moments of the multinomial distribution, as well as explicit formulas for all the non-central moments up to order 8 and all the central moments up to order 4. Our work expands on the results in [2], where the central moments were calculated up to order 4. It also complements the general formula for the (joint) factorial moments from [1] and the explicit formulas for some of the lower-order (mixed) cumulants that were presented in [3].

Author Contributions

All contributions were made by the sole author of the article, F.O. The author has read and agreed to the published version of the manuscript.

Funding

This research was funded by a postdoctoral fellowship from the NSERC (PDF) and a postdoctoral fellowship supplement from the FRQNT (B3X).

Acknowledgments

We thank the three referees for their comments.

Conflicts of Interest

The author declares no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
i.i.d.independent and identically distributed

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