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  "slug": "davis-yin-gap-sharpness",
  "title": "Exact finite-horizon gap constants and non-attainment for Davis–Yin splitting",
  "shortTitle": "The exact Davis–Yin gap constant",
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  "oneLine": "An exact decomposition gives the sharp averaged comparison-gap coefficient across the full unrelaxed step range and explains why the worst case is usually approached but not attained.",
  "abstract": "This research candidate determines the exact worst-case coefficient of an averaged primal-dual comparison gap for Davis–Yin three-operator splitting over the full unrelaxed range 0<alpha L<2. After K iterations the sharp denominator is alpha(K+1-alpha L) for the sum of squared primal and scaled dual distances, and the exact coefficient is half as large in the natural splitting metric. A signed identity decomposes the shortfall into convexity slacks, smooth-interpolation deficits, variances and squares. An explicit two-dimensional family approaches both constants. Equality is attainable for one iteration but, for K>=2, no finite-dimensional positive-denominator instance attains either open-range supremum. At the endpoint alpha L=2 and K>=2 the exact denominator becomes alpha(K-1); strict convexity of f rules out attainment, while the general endpoint attainment question remains open. The sharpness quantifier permits arbitrary admissible comparison pairs and does not establish saddle-pair sharpness, last-iterate rates, objective-error rates or lower bounds for other algorithms.",
  "datePublished": "2026-10-20",
  "dateModified": "2026-10-20",
  "version": "1.0.0-candidate",
  "doi": "10.5281/zenodo.23505130",
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  "authors": [
    "Anonymous"
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    "Davis–Yin splitting",
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    "For 0<alpha L<2 the exact denominator after K iterations is K+1-alpha L.",
    "One-step equality is attainable, but for K>=2 no finite-dimensional positive-denominator instance attains either open-range supremum.",
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  "relatedWorks": [
    {
      "citation": "Davis and Yin (2017), A three-operator splitting scheme and its optimization applications.",
      "url": "https://doi.org/10.1007/s10957-016-0917-z",
      "doi": "10.1007/s10957-016-0917-z"
    },
    {
      "citation": "Nguyen et al. (2025), source averaged-gap bound and tightness conjecture.",
      "url": "https://arxiv.org/abs/2506.23475",
      "doi": "10.48550/arXiv.2506.23475"
    },
    {
      "citation": "Taylor, Hendrickx and Glineur (2017), exact worst-case performance of first-order methods.",
      "url": "https://doi.org/10.1137/16M1081143",
      "doi": "10.1137/16M1081143"
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