  We show that the value function of an optimal stopping game driven by a
one-dimensional diffusion can be characterised using a modification of the
Legendre transformation if and only if the optimal stopping game exhibits a
Nash equilibrium (i.e. a saddle point of the optimal stopping game exists).
This result is an analytical complement to the results in Peskir, G. (2012) A
Duality Principle for the Legendre Transform. Journal of Convex Analysis,
19(3), 609-630 where the `duality' between a concave-biconjugate which is
modified to remain below an upper barrier and a convex-biconjugate which is
modified to remain above a lower barrier is proven by appealing to the
probabilistic result in Peskir, G. (2008) Optimal stopping games and Nash
equilibrium. Theory Probab. 53 (558-571). The main contribution of this paper
is to show that, in this special case, the semi-harmonic characterisation of
the value function may be proven using only results from convex analysis.
