The bounded connectivity conjecture for breadth-critical tangles

Let k2k\geq 2 and let MM be a matroid with a breadth-critical tangle T\mathcal{T} of order at least kk. A matroid is (s0,s1,,st)(s_0,s_1,\ldots,s_t)-connected if, whenever FE(M)F\subseteq E(M) has λ(F)=i\lambda(F)=i for i{0,1,,t}i\in\{0,1,\ldots,t\}, either Fsi|F|\leq s_i or E(M)Fsi|E(M)-F|\leq s_i. The bounded connectivity conjecture. There is an infinite sequence (s0,s1,s2,)(s_0,s_1,s_2,\ldots) such that, for all k2k\geq 2, if T\mathcal{T} is a breadth-critical tangle of order at least kk in MM, then MM is (s0,s1,,sk2)(s_0,s_1,\ldots,s_{k-2})-connected. This would extend the established fact that breadth-critical tangles of order at least 44 occur in weakly 44-connected, equivalently (0,1,4)(0,1,4)-connected, matroids; the existence of a uniform sequence for all orders remains open.

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Primary source

Nick Brettell, Susan Jowett, James Oxley, Charles Semple and Geoff Whittle, “What is a 4-connected matroid?”, arXiv:2310.08832 (2025).

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