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The S_4 character table by rim-hook recursion

Example

The Murnaghan–Nakayama rule (The Murnaghan–Nakayama character rule) computes the full ordinary character table of S4. With rows λ=(4),(3,1),(2,2),(2,1,1),(14) and columns the classes (14),(2,1,1),(2,2),(3,1),(4) (of sizes 1,6,3,8,6),

χ(4)11111χ(3,1)31−10−1χ(2,2)202−10χ(2,1,1)3−1−101χ(14)1−111−1

(for example χ(2,2) on the 4-cycle class is 0 because (2,2) is not a hook (The value of an irreducible character on a full cycle is supported on hooks), and χ(3,1)(3,1)=0, χ(3,1)(4)=−1 by the same rule). Checking the first orthogonality relation (The first orthogonality relation for irreducible complex characters) row by row gives, for instance, ⟨χ(3,1),χ(2,2)⟩=124(1⋅3⋅2+6⋅1⋅0+3⋅(−1)⋅2+8⋅0⋅(−1)+6⋅(−1)⋅0)=0, and every row has norm 1.

Facts & Assumptions

Given: The partitions of 4, the conjugacy classes of S4 indexed by cycle type with sizes ∣Cρ∣ (The conjugacy classes of Sn are indexed by the tuples (c1,…,cn) with ∑kck=n, If σ∈Sn has ck cycles of length k, then ∣CSn(σ)∣=∏k=1nkckck!), and for every partition of an integer the character χλ of the complex Specht module Sλ (Column antisymmetrizers, polytabloids, and Specht modules).

[F1]

Murnaghan–Nakayama rule: for a cycle c of length r on a specified set of points fixed by σ (a specified fixed point for length 1), χλ(σc)=∑λ/μ(−1)ht⁡(λ/μ)χμ(σ), summed over the removable border strips of λ of size r, the empty sum being 0 (The Murnaghan–Nakayama character rule).

[F2]

Hook lengths and dimensions: the hook lengths of (3,1),(2,2),(2,1,1),(14) are 4,2,1,1; 3,2,2,1; 4,2,1,1; 4,3,2,1, and the dimensions are fλ=4!/∏h(x), namely 1,3,2,3,1 for (4),(3,1),(2,2),(2,1,1),(14) (Hook, arm, leg, and hook length of a box, The hook length formula).

[F4]

Standard inner product ⟨φ,ψ⟩=1∣G∣∑g∈Gφ(g)ψ(g)‾ (The standard inner product on cf(G)), and the first orthogonality relation ⟨χλ,χμ⟩=δλμ for irreducible characters (The first orthogonality relation for irreducible complex characters); the character table of a finite group has rows the irreducible characters and columns the classes (The character table of a finite group).

[F5]

For the empty partition χ∅ is the trivial character of S0, so χ∅(1)=1 (Column antisymmetrizers, polytabloids, and Specht modules).

Verification

technique · direct
1.1F1F5algebra

Row (4): for each part r of the cycle type the unique removable r-strip of the row (4) is horizontal and has height 0, leaving the row (4−r), so iterating the rule [F1] gives χ(4)=1 on all five classes. Row (14) is the sign character: the unique r-strip of the column is vertical of height r−1, so iterating [F1] gives χ(14)(ρ)=∏i(−1)ri−1 for the parts ri of the cycle type, that is the values 1,−1,1,1,−1 on (14),(2,1,1),(2,2),(3,1),(4).

1.2F1F2F5algebra

Row (3,1): by [F2] the dimension is 4!/(4⋅2⋅1⋅1)=3; on (2,1,1) the only 2-strip removes the domino {(1,2),(1,3)} and leaves (1,1) with height 0, so the value is χ(1,1)(1,1)=1; on (2,2) the same strip gives χ(1,1)(2)=−1; on (3,1) the hook lengths 4,2,1,1 contain no 3, so there is no 3-strip and the value is 0; on (4) the whole diagram (3,1) is the unique 4-strip, of height 1, so the value is −χ∅=−1. This gives the row 3,1,−1,0,−1.

1.3F1F2F5algebra

Row (2,2): the two 2-strips are the bottom row to (2) (height 0) and the right column to (1,1) (height 1), so on (2,1,1) the value is χ(2)(1,1)−χ(1,1)(1,1)=1−1=0, on (2,2) it is χ(2)(2)−χ(1,1)(2)=1−(−1)=2; on (3,1) the unique 3-strip (leaving the single node (1)) has height 1 and gives −χ(1)(1)=−1; on (4) the hook lengths 3,2,2,1 contain no 4, so the value is 0; the dimension is 2 by [F2]. Row (2,1,1): the unique 2-strip removes the vertical domino {(2,1),(3,1)} and leaves (2) with height 1, so the values on (2,1,1) and (2,2) are −χ(2)(1,1)=−1 and −χ(2)(2)=−1; there is no 3-strip because the hook lengths are 4,2,1,1; the whole diagram is the unique 4-strip, of height 2, giving χ∅=1 on (4); the dimension is 3 by [F2]. This gives the rows 2,0,2,−1,0 and 3,−1,−1,0,1.

2.1step 1.1step 1.2step 1.3F3F4algebra∎

With the class sizes 1,6,3,8,6 [F3] and the standard inner product [F4], the five rows have norms 124(1+6+3+8+6)=1 for (4) and (14), 124(9+6+3+0+6)=1 for (3,1) and (2,1,1), and 124(4+0+12+8+0)=1 for (2,2); pairwise orthogonality holds as in the displayed computation ⟨χ(3,1),χ(2,2)⟩=124(6−6)=0, and likewise ⟨χ(2,2),χ(2,1,1)⟩=124(1⋅2⋅3+6⋅0⋅(−1)+3⋅2⋅(−1)+8⋅(−1)⋅0+6⋅0⋅1)=0, the remaining pairs checking in the same way. Hence the five rows computed by the recursion are exactly the character table of S4 displayed in the statement, with the rows orthonormal with respect to the class sizes of [F3].

Remarks

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