Function tangent 

pub fn tangent(x: &[f64]) -> Evaluation

The tangent problem: the sphere with Σ xᵢ ≥ n, as g = n − Σ xᵢ ≤ 0, a constraint along no variable’s axis. By symmetry and Lagrange’s condition 2 xᵢ = λ, the minimum is n, at x = 1.