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A partition is determined by the multiset of its node contents

Statement

Let λ,μ⊢n. If the multiset of contents {c(x):x∈[λ]} equals the multiset {c(y):y∈[μ]}, then λ=μ.

Facts & Assumptions

Given: Partitions λ,μ⊢n; for a partition ν⊢n we write [ν] for its Young diagram, ν′ for its conjugate, and νj′=#{i:νi≥j} for the height of column j (Partitions, English diagrams, and conjugation).

[F1]

A node of [ν] is a pair (r,c) with r,c≥1 and c≤νr; its content is c(r,c)=c−r; the rows of [ν] are weakly decreasing (Partitions, English diagrams, and conjugation).

[F2]

The content of a node and the content vector are as defined in The content of a node and the content vector of a standard tableau; in particular the content map is c(r,c)=c−r on nodes.

Proof

technique · direct
1.1F1F2algebra

For a partition ν⊢n and an integer t≥0 put nt(ν):=#{x∈[ν]:c(x)=t}. A node of content t has the form (i,i+t) with i≥1, and it lies in [ν] exactly when i+t≤νi, that is νi−i≥t; hence nt(ν)=#{i≥1:νi−i≥t} for every t≥0, the count being finite and equal to 0 for t>n.

1.2F1F2algebra

Similarly, for an integer s≥0 put n−s(ν):=#{x∈[ν]:c(x)=−s}. A node of content −s is (j+s,j) with j≥1; it lies in [ν] exactly when j+s≤νj′, that is νj′−j≥s. Hence n−s(ν)=#{j≥1:νj′−j≥s} for every s≥0.

1.3F1F2algebra

The partition ν is recovered from the pair of strictly decreasing sequences a1>a2>⋯>ad and b1>b2>⋯>bd by the formula νr=(ar+1)⋅[r≤d]+#{c:1≤c<r, bc≥r−c} for every row index r. Indeed, for each diagonal node (i,i)∈[ν] with i≤d let Ai:={(i,c):i≤c≤νi} be its arm and Li:={(r,i):i≤r≤νi′} its leg; arms and legs have sizes ai+1 and bi+1, and the d hooks Ai∪Li partition [ν], because a node (r,c) with c≥r lies in the arm Ar and a node (r,c) with c<r lies in the leg Lc. Counting row r therefore gives νr=∣Ar∣+#{c<r:(r,c)∈Lc}=(ar+1)⋅[r≤d]+#{c<r:bc≥r−c}, since (r,c)∈Lc means c≤r≤νc′=c+bc.

2.1step 1.1step 1.2F1algebra

For each row index i put ai:=νi−i, and for each column index j put bj:=νj′−j. The row lengths are weakly decreasing, so ai+1<ai for all i; the column heights are weakly decreasing as well, so bj+1<bj for all j. Moreover ai≥0 exactly for the diagonal rows i with (i,i)∈[ν], and bj≥0 exactly for the diagonal columns j with (j,j)∈[ν], so the two multisets A(ν):={ai:ai≥0} and B(ν):={bj:bj≥0} have a common cardinality d(ν), the number of diagonal nodes. By steps 1.1 and 1.2 the numbers nt(ν), t≥0, determine the multiplicity of every value t≥0 among the ai, namely nt(ν)−nt+1(ν), and hence determine the multiset A(ν) together with d(ν); likewise the numbers n−s(ν), s≥0, determine B(ν).

3.1step 2.1step 1.3given∎

Assume now that the multiset of contents of [λ] equals that of [μ]. Then nt(λ)=nt(μ) for every integer t; by steps 1.1 and 1.2 this forces A(λ)=A(μ) and B(λ)=B(μ), including the common cardinality d. Writing both multisets as strictly decreasing sequences a1>⋯>ad and b1>⋯>bd, step 1.3 computes the row lengths of λ and μ by the same formula from the same data, so all row lengths agree and λ=μ.

Remarks

  • Why the diagonal data are Frobenius coordinates. The numbers a1>⋯>ad and b1>⋯>bd are the arm and leg lengths of the diagonal nodes, the Frobenius coordinates of ν; the formula of step 1.3 is the usual reconstruction of a partition from them. The lemma says that the content multiset, which records the ai and bj through the diagonal counts of steps 1.1 and 1.2, is equivalent to that data.

  • Sharper statement. The proof shows the two multisets A(ν) and B(ν) separately, not merely their union; both are needed, since the nonnegative and negative contents determine the arms and the legs respectively.

Depends on

Used by

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Sources