Diophantine approximation with constraints

Following Schmidt, Thurnheer and Bugeaud-Kristensen, we study how Dirichlet’s theorem on linear forms needs to be modified when one requires that the vectors of coefficients of the linear forms make a bounded acute angle with respect to a fixed proper non-zero subspace $V$ of ${\mathbb{R}}^n$. Assuming that the point of ${\mathbb{R}}^n$ that we are approximating has linearly independent coordinates over $\mathbb{Q}$, we obtain best possible exponents of approximation which surprisingly depend only on the dimension of $V$. Our estimates are derived by reduction to a result of Thurnheer, while their optimality follows from a new general construction in parametric geometry of numbers involving angular constraints.