Reversal-Wilf conjecture for contraction containment in labeled trees

Let Tm\mathcal T_m denote the set of labeled trees on [m]={1,,m}[m]=\{1,\ldots,m\}. For TTnT\in\mathcal T_n and UTmU\in\mathcal T_m, write TUT\preceq U if UU displays TT, meaning that TT can be obtained from UU by contracting connected fibers and then standardizing the survivor labels. Define the support count by

μT(m)=#{UTm:TU}.\mu_T(m)=\#\{U\in\mathcal T_m:T\preceq U\}.

Let RnR_n be label reversal, in+1ii\mapsto n+1-i. Reversal-Wilf conjecture. For T1,T2TnT_1,T_2\in\mathcal T_n, if

μT1(m)=μT2(m)(mn),\mu_{T_1}(m)=\mu_{T_2}(m)\qquad(m\ge n),

then T2=T1T_2=T_1 or T2=Rn(T1)T_2=R_n(T_1). Reversal of the ambient label set already gives μT(m)=μRn(T)(m)\mu_T(m)=\mu_{R_n(T)}(m) for all mm, so the conjecture asserts that these are the only equal support-count sequences. It asks whether contraction containment among labeled trees has no Wilf-equivalences beyond label reversal.

Sources & referencesView supporting material

Primary source

Levi Segal, “Structure, Enumeration, and star avoidance for contraction containment in labeled trees”, arXiv:2606.07668 (2026).

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