The theorem states: for all real x with x > 0, Jcost(x) = 0 if and only if x = 1.
In standard notation, let J(x) = (x + x^{-1})/2 - 1 for x > 0. The claim is that J(x) = 0 precisely when x = 1.
Proof (translated from Lean):
- (⇒) Assume Jcost(x) = 0. Suppose x eq 1. Then Jcost(x) > 0, a contradiction. Hence x = 1.
- (⇐) Assume x = 1. Then Jcost(1) = 0 by direct evaluation.
This shows the zero-cost set on ℝ^{+} is exactly the singleton {1}. The same uniqueness is restated in cost_zero_set_has_one_member for any two positive elements.