Failure of the pointwise-convergence hypothesis in the non-compact sufficiency theorem

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Let (X,d)(X,d) be a non-compact metric space, let f:X→Rf:X\to\mathbb{R} be bounded and continuous, and let p:X×R→Rp:X\times\mathbb{R}\to\mathbb{R} be a convex extension of ff. Suppose there exist maximizing sequences {xn}\{x_n\} and {yn}\{y_n\} of ff, a δ>0\delta>0, and sequences tn→0+t_n\to0+ and sn→0−s_n\to0- such that

lim sup⁡n→∞pt+(xn,tn)≥0,lim inf⁡n→∞pt−(yn,sn−)≤0.\limsup_{n\to\infty}p_{t+}(x_n,t_n)\geq0,\qquad \liminf_{n\to\infty}p_{t-}(y_n,s_n-)\leq0.

Pointwise-convergence failure conjecture. The conclusion that pp is a Birkhoff-James extension fails if the assumption that p(xn, )p(x_n,\,\rule{3mm}{0.15mm}) and p(yn, )p(y_n,\,\rule{3mm}{0.15mm}) converge pointwise to a common function gg on (−δ,δ)(-\delta,\delta) is removed.

The claim asserts that the pointwise-convergence hypothesis in the preceding sufficient condition is essential: without it, the remaining assumptions need not force pp to be a Birkhoff-James extension. The source provides no resolution, so the conjecture is recorded as open.

References

Primary source

Saptak Bhattacharya, “Birkhoff-James extensions of continuous functions on metric spaces”, arXiv:2108.12576 (2021).

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