A full complexity dichotomy for immanant families

Given an integer n ≥ 1 and an irreducible character χλ of Sn for some partition λ of n, the immanant immλ:ℂn× n→ℂ maps matrices A∈ℂn× n to immλ(A)=∑π∈ Snχλ(π)∏i=1nAi,π(i). Important special cases include the determinant and permanent, which are obtained from the sign and trivial character, respectively.

It is known that immanants can be evaluated in polynomial time for characters that are “close” to the sign character: Given a partition λ of n with s parts, let b(λ):=n−s count the boxes to the right of the first column in the Young diagram of λ. For a family of partitions Λ, let b(Λ) := maxλ∈Λb(λ) and write Imm(Λ) for the problem of evaluating immλ(A) on input A and λ∈Λ. On the positive side, if b(Λ)<∞, then Imm(Λ) is known to be polynomial-time computable. This subsumes the case of the determinant. Conversely, if b(Λ)=∞, then previously known hardness results suggest that Imm(Λ) cannot be solved in polynomial time. However, these results only address certain restricted classes of families Λ.

In this paper, we show that the assumption FPT≠ #W[1] from parameterized complexity rules out polynomial-time algorithms for Imm(Λ) for any computationally reasonable family of partitions Λ with b(Λ)=∞. We give an analogous result in algebraic complexity under the assumption VFPT≠ VW[1]. Furthermore, if b(λ) even grows polynomially in Λ, we show that Imm(Λ) is hard for #P and VNP. This concludes a series of partial results on the complexity of immanants obtained over the last 35 years.