Consider the Slutsky equation:

x₁= x₁^s + x₁ⁿ

x₁ (p'₁, m) x₁ (p₁, m) = [x₁ (p'₁, m') x₁ (p₁, m)] + [x₁ (p'₁ , m) x₁ (p'₁ , m' )]

What condition would be necessary with regards to this equation for a consumer to have a perfectly vertical demand curve?