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Apr 7, 2020 at 14:28 comment added Pierre PC I don't know that it's commonly used. If the sum of non-negative terms over an arbitrary set is convergent, then only countably many of them are positive (S = indices of terms $>\varepsilon$); a compact operator has at most countably many eigenvalues, with at most one limit point (S = eigenvalues with modulus $>\varepsilon$); a discrete closed set of $\mathbb R^d$ is countable (S = elements with norm $\leq R$). I guess it is useful enough to be found here and there, but not sophisticated enough to have a name.
Apr 7, 2020 at 8:10 comment added Anonymous amateur Thanks so much for the detailed answer. One soft question: Is this particular trick of considering $S_{\epsilon, R}$ (or its variations) commonly used in analysis? I feel that it may be useful in certain context....
Apr 7, 2020 at 8:06 vote accept Anonymous amateur
Apr 4, 2020 at 21:48 history answered Pierre PC CC BY-SA 4.0