A full complexity dichotomy for immanant families
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Given an integer n≥ 1 and an irreducible character χ_λ of S_n for some partition λ of n, the immanant imm_λ:ℂ^n× n→ℂ maps matrices A∈ℂ^n× n to imm_λ(A)=∑_π∈ S_nχ_λ(π)∏_i=1^nA_i,π(i). Important special cases include the determinant and permanent, which are the immanants associated with 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):=max_λ∈Λb(λ) and write Imm(Λ) for the problem of evaluating imm_λ(A) on input A and λ∈Λ. If b(Λ)<∞, then Imm(Λ) is known to be polynomial-time computable. This subsumes the case of the determinant. On the other hand, 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 parameterized complexity assumption FPT ≠ #W[1] 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.
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