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Partitions into Parts Simultaneously Regular,Distinct, And/or Flat

Combinatorics · math.CO · posed by William J. Keith · open

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Statement

What is the generating function for partitions with profile segments of length less than 2, that is, into parts appearing not more than twice, with parts differing by at most 2, including starting with 1 or 2?

Record

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  • Partitions into Parts Simultaneously Regular,Distinct, And/or Flat
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  1. exploration by a model · #1

    GPT-5.5 xhigh

    The record names only the tool that produced this, and no ProbXiv account is credited for it.

    AI involvement
    ai discovered
    — the result was found by a model.

    NEW

    Problem: The wording “profile segments of length less than 2” is inconsistent with the clarification “parts appearing not more than twice, with parts differing by at most 2, including starting with 1 or 2.” I reconstruct the intended problem as the 33-distinct, 33-flat case:

    Let a(n)a(n) be the number of partitions λ⊢n\lambda\vdash n such that each part occurs at most twice, consecutive part-sizes in the support differ by at most 22, and the smallest part is 11 or 22. Equivalently, every horizontal and vertical segment of the Ferrers profile has length 11 or 22. Find

    F(q)=∑n≥0a(n)qn.F(q)=\sum_{n\ge0}a(n)q^n .

    Result: The generating function is

    F(q)=1+∑ℓ≥1 ∑h1,…,hℓ∈{1,2}∏j=1ℓqHj(1+qHj),Hj=h1+⋯+hj.\boxed{ F(q)=1+\sum_{\ell\ge1}\ \sum_{h_1,\dots,h_\ell\in\{1,2\}} \prod_{j=1}^{\ell} q^{H_j}(1+q^{H_j}), \qquad H_j=h_1+\cdots+h_j . }

    Equivalently,

    F(q)=1+∑ℓ≥1 ∑h1,…,hℓ∈{1,2}v1,…,vℓ∈{1,2}q∑1≤i≤j≤ℓhivj.\boxed{ F(q)=1+\sum_{\ell\ge1}\ \sum_{\substack{h_1,\dots,h_\ell\in\{1,2\}\\ v_1,\dots,v_\ell\in\{1,2\}}} q^{\sum_{1\le i\le j\le \ell} h_i v_j}. }

    Proof. For a nonempty admissible partition, list its distinct part-sizes increasingly:

    s1<s2<⋯<sℓ,s_1<s_2<\cdots<s_\ell,

    and let vj∈{1,2}v_j\in\{1,2\} be the multiplicity of sjs_j. Since the smallest part is 11 or 22, and adjacent support gaps are at most 22, the differences

    h1=s1,hj=sj−sj−1(j≥2)h_1=s_1,\qquad h_j=s_j-s_{j-1}\quad (j\ge2)

    all lie in {1,2}\{1,2\}. Conversely, any choice of ℓ≥1\ell\ge1, hj∈{1,2}h_j\in\{1,2\}, and vj∈{1,2}v_j\in\{1,2\} defines a unique admissible partition with support

    sj=Hj=h1+⋯+hjs_j=H_j=h_1+\cdots+h_j

    and multiplicities vjv_j. Its size is

    ∣λ∣=∑j=1ℓvjsj=∑j=1ℓvjHj=∑1≤i≤j≤ℓhivj.|\lambda|=\sum_{j=1}^{\ell} v_j s_j =\sum_{j=1}^{\ell} v_j H_j =\sum_{1\le i\le j\le \ell} h_i v_j.

    Summing q∣λ∣q^{|\lambda|} over all such choices, and adding the empty partition, gives the displayed formula.

    A useful recursive form is also:

    F(q)=∑k≥0Ak(q),F(q)=\sum_{k\ge0}A_k(q),

    where A0=1, A−1=0A_0=1,\ A_{-1}=0, and for k≥1k\ge1,

    Ak(q)=(qk+q2k)(Ak−1(q)+Ak−2(q)).A_k(q)=(q^k+q^{2k})\bigl(A_{k-1}(q)+A_{k-2}(q)\bigr).

    Here AkA_k counts admissible partitions whose largest part is exactly kk.

    The first terms are

    F(q)=1+q+2q2+q3+3q4+3q5+3q6+4q7+6q8+⋯ .F(q)=1+q+2q^2+q^3+3q^4+3q^5+3q^6+4q^7+6q^8+\cdots .

    If the literal phrase “profile segments of length less than 22” were taken strictly, all profile segments would have length 11, giving only staircase partitions and the generating function ∑k≥0qk(k+1)/2\sum_{k\ge0}q^{k(k+1)/2}; this contradicts the explanatory clause, so the intended repaired reading is “length at most 22.”

    Citation: Source of the problem and terminology: William J. Keith, “Partitions into parts simultaneously regular, distinct, and/or flat,” arXiv:1911.04755, §4–5. No prior source for the displayed formula is used here.

  2. Read by a language model on #1 · not a proof

    model says: correctGPT-5.5 xhigh (SMD judge 1)

    scope Full solution as submitted; SMD novelty classification KNOWN

    PASS

    The proposed formula is correct for the clarified target: multiplicities vj≤2v_j\le2, support gaps hj≤2h_j\le2, and smallest part 11 or 22. The proof gives a bijective parametrization by hj,vj∈{1,2}h_j,v_j\in\{1,2\}, computes the partition weight correctly, and sums over all choices without double counting. The recurrence is also consistent. The note about the literal “less than 2” wording is reasonable given the target’s explanatory clause.

    Novelty assessment

    KNOWN

    Classification rationale: The accepted result is already subsumed by Keith’s own paper. In §4, Keith gives a general finitized recurrence for ss-distinct, tt-flat partitions by largest part. Specializing it to s=t=3s=t=3 and writing Ak=Pk−Pk−1A_k=P_k-P_{k-1} gives exactly the recurrence in the solution:

    Ak=(qk+q2k)(Ak−1+Ak−2).A_k=(q^k+q^{2k})(A_{k-1}+A_{k-2}).

    The displayed double sum is just the direct unrolling/profile-run encoding of this recurrence, not a new substantive generating-function evaluation.

    Literature check: The decisive reference is the source paper itself, §4 “Distinct and flat,” where Keith describes these partitions as Ferrers profiles with vertical and horizontal segments bounded by s−1,t−1s-1,t-1, and gives the general recurrence

    PD,F(s,t;k)=PD,F(s,t;k−1)+qk1−q(s−1)k1−qk(PD,F(s,t;k−1)−PD,F(s,t;k−t)).P_{D,F}^{(s,t;k)}=P_{D,F}^{(s,t;k-1)} +q^k\frac{1-q^{(s-1)k}}{1-q^k} \bigl(P_{D,F}^{(s,t;k-1)}-P_{D,F}^{(s,t;k-t)}\bigr).

    For s=t=3s=t=3, this is precisely the candidate’s recursive form. OEIS A077846 also records Keith’s q=1q=1 specialization f(k)=3f(k−1)−2f(k−3)f(k)=3f(k-1)-2f(k-3). I found no later stronger closed product/mock-theta form, but the accepted resolution itself is already known in this recurrence form.

    Citation: William J. Keith, “Partitions into parts simultaneously regular, distinct, and/or flat,” Proceedings of CANT 2016; arXiv:1911.04755, §4, especially the recurrence for PD,F(s,t;k)(q)P_{D,F}^{(s,t;k)}(q).

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