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Littlewood-Richardson Coefficient


A Littlewood-Richardson coefficient c_(munu)^lambda is the nonnegative integer coefficient in the expansion of a product of Schur polynomials:

 s_mus_nu=sum_(lambda)c_(munu)^lambdas_lambda.
(1)

The partitions satisfy |lambda|=|mu|+|nu| when the coefficient is nonzero. The identity is understood in sufficiently many variables, or in the ring of symmetric functions. Equivalently, for mu|-k and nu|-l, c_(munu)^lambda is the multiplicity of the Specht module S^lambda in the representation of S_(k+l) induced from S^mu tensor S^nu for S_k×S_l (Macdonald 1995). Thus these coefficients concern induction between different symmetric groups, whereas a Kronecker coefficient concerns a vector space tensor product of representations of the same symmetric group. For example, s_((1))^2=s_((2))+s_((1,1)).

The Littlewood-Richardson rule counts the semistandard skew tableaux of shape lambda/mu and content nu whose row-reading word is a lattice word. Rows are read from right to left, beginning with the top row. Lattice means that in every initial segment the number of occurrences of i is at least the number of occurrences of i+1, for every positive integer i (Macdonald 1995).

Write f^rho for the dimension of the Specht module indexed by a partition rho of size r, and define its dimension deficit by

 Delta(rho)=ln(sqrt(r!))/(f^rho).
(2)

For n=k+l and c_(munu)^lambda>0, put delta=ln(sqrt((n; k))/c_(munu)^lambda). Slonim (2026) reports that, for every fixed 0<epsilon<=1/2, there are positive constants a_epsilon and b_epsilon such that

 Delta(lambda)+Delta(mu)+Delta(nu)<=a_epsilon+b_epsilondeltalnn
(3)

whenever n>=2 and k,l>=epsilonn. The requirement that the coefficient be positive is essential to this finite-deficit statement.

Large dimensions alone do not force positivity. For every fixed A>0, Slonim (2026) reports an A^'>0 such that, for every sufficiently large even n and every pair of partitions mu and nu of n/2 with f^mu,f^nu>=sqrt((n/2)!)exp(-Asqrt(n/2)), there is a partition lambda of n with f^lambda>=sqrt(n!)exp(-A^'sqrt(n)) and c_(munu)^lambda=0. This assertion concerns vanishing coefficients, not an arbitrarily small positive coefficient.

For |lambda|=|mu|+|nu|, the same work reports c_(munu)^lambda<=f^mu. When f^mu>1 and the diagram of nu is contained in that of lambda, equality holds iff the skew shape lambda/nu has at most one box in each row and each column. For a shape of that kind, equality holds for every partition mu of its size.

Let D(k)=max_(mu|-k)f^mu and let C(n,k) be the largest c_(munu)^lambda over partitions of sizes n, k, and n-k. Slonim (2026) reports the exact stabilization threshold

 C(n,k)=D(k)<==>n>=(k(k+1))/2
(4)

for k>=3 and n>=k. The exceptions are k=0, k=1, and k=2, for which C(n,k)=D(k)=1 for every n>=k, using the empty partition at k=0. The equality construction uses a skew shape consisting of k boxes with at most one in each row and each column. For example, at n=k(k+1)/2+r with r>=0, take lambda=(k+r,k-1,...,1) and nu=(k-1+r,k-2,...,1).

Slonim attributes the mathematical statements and proofs to ChatGPT 6 Pro and explicitly states that the technical proofs have not been independently verified.


See also

Kronecker Coefficient, Schur Polynomial, Skew Tableau, Specht Module, Young Tableau

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References

Macdonald, I. G. Symmetric Functions and Hall Polynomials, 2nd ed. Oxford, England: Oxford University Press, 1995.Slonim, Z. "Kronecker Coefficients of Large Degree and Littlewood-Richardson Inequalities." 8 Nov 2026. https://arxiv.org/abs/2611.11593v1.

Cite this as:

Weisstein, Eric W. "Littlewood-Richardson Coefficient." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Littlewood-RichardsonCoefficient.html

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