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For an integer ℓ≥2, let U(ℓ) be the class of matroids with no U2,ℓ+2-minor. A matroid in U(ℓ) is extremal if it is simple and has no simple rank-preserving single-element extension in U(ℓ). An amalgam of two matroids is a simultaneous extension of both on the union of the two ground sets. We study amalgams of extremal matroids in U(ℓ): we determine which amalgams are in U(ℓ) and which are extremal...
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