In mathematics, Delta-convergence, or Δ-convergence, is a mode of convergence in metric spaces, weaker than the usual metric convergence, and similar to (but distinct from) the weak convergence in Banach spaces. In Hilbert space, Delta-convergence and weak convergence coincide. For a general class of spaces, similarly to weak convergence, every bounded sequence has a Delta-convergent subsequence. Delta convergence was first introduced by Teck-Cheong Lim,^[1] and, soon after, under the name of almost convergence, by Tadeusz Kuczumow.^[2]

A sequence ${\displaystyle (x_{k})}$ in a metric space ${\displaystyle (X,d)}$ is said to be Δ-convergent to ${\displaystyle x\in X}$ if for every ${\displaystyle y\in X}$, ${\displaystyle \limsup(d(x_{k},x)-d(x_{k},y))\leq 0}$.

If ${\displaystyle X}$ is a uniformly convex and uniformly smooth Banach space, with the duality mapping ${\displaystyle x\mapsto x^{*}}$ given by ${\displaystyle \|x\|=\|x^{*}\|}$, ${\displaystyle \langle x^{*},x\rangle =\|x\|^{2}}$, then a sequence ${\displaystyle (x_{k})\subset X}$ is Delta-convergent to ${\displaystyle x}$ if and only if ${\displaystyle (x_{k}-x)^{*}}$ converges to zero weakly in the dual space ${\displaystyle X^{*}}$ (see ^[3]). In particular, Delta-convergence and weak convergence coincide if ${\displaystyle X}$ is a Hilbert space.

Coincidence of weak convergence and Delta-convergence is equivalent, for uniformly convex Banach spaces, to the well-known Opial property^[3]

The Delta-compactness theorem of T. C. Lim^[1] states that if ${\displaystyle (X,d)}$ is an asymptotically complete metric space, then every bounded sequence in ${\displaystyle X}$ has a Delta-convergent subsequence.

The Delta-compactness theorem is similar to the Banach–Alaoglu theorem for weak convergence but, unlike the Banach-Alaoglu theorem (in the non-separable case) its proof does not depend on the Axiom of Choice.

An asymptotic center of a sequence ${\displaystyle (x_{k})_{k\in \mathbb {N} }}$, if it exists, is a limit of the Chebyshev centers ${\displaystyle c_{n}}$ for truncated sequences ${\displaystyle (x_{k})_{k\geq n}}$. A metric space is called asymptotically complete, if any bounded sequence in it has an asymptotic center.

Condition of asymptotic completeness in the Delta-compactness theorem is satisfied by uniformly convex Banach spaces, and more generally, by uniformly rotund metric spaces as defined by J. Staples.^[4]

• William Kirk, Naseer Shahzad, Fixed point theory in distance spaces. Springer, Cham, 2014. xii+173 pp.
• G. Devillanova, S. Solimini, C. Tintarev, On weak convergence in metric spaces, Nonlinear Analysis and Optimization (B. S. Mordukhovich, S. Reich, A. J. Zaslavski, Editors), 43–64, Contemporary Mathematics 659, AMS, Providence, RI, 2016.