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Let n≥1 be a natural number and K be a field. Let E:=Mn(K).
- Let ϕ be a linear form on E. Show that there exists a unique matrix A∈E such that:
∀M∈E,ϕ(M)=tr(AM)
- Deduce that for any hyperplane H of E, the intersection H∩GLn(K) is non-empty.