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Absorbing polylogarithmic overhead above the I/O exponent

Proved
WordRAM.APSPBridge.polylog_absorption

by wurtle · Nov 2, 2026 · Mathlib 0df444a (Lean v4.33.1)

asymptoticsfine-grained-complexityruntime-transferwordram

Let TTT be a runtime function with T(n)=O(nr)T(n)=O(n^r)T(n)=O(nr) in the platform's rational-exponent sense, let qqq be the I/O exponent, and fix natural numbers ddd, KKK and CCC. Then for every rational r′>max⁡(r,q)r'>\max(r,q)r′>max(r,q), the transfer bound

C(T(n)+K(n+1)q+1)(⌊log⁡2n⌋+1)dC\bigl(T(n)+K(n+1)^q+1\bigr)\bigl(\lfloor\log_2 n\rfloor+1\bigr)^dC(T(n)+K(n+1)q+1)(⌊log2​n⌋+1)d

is again O(nr′)O(n^{r'})O(nr′). In words: a polylogarithmic overhead is absorbed by any positive exponent gap above both the algorithm's exponent and the I/O exponent. This is the asymptotic step that turns the uniform runtime-transfer theorem into a fine-grained threshold equivalence.

Preamble
import Definitions.Def_WordRAM_APSP_Bridge

set_option autoImplicit false
Formal statement
theorem WordRAM.APSPBridge.polylog_absorption (d K q C : Nat) (r : Rat) (T : Nat → Nat) (hT : TrulySubcubicAPSP.BigO T r) : ∀ r' : Rat, max r (q : Rat) < r' → TrulySubcubicAPSP.BigO (fun n => C * (T n + WordRAM.APSPBridge.ioBound K q n + 1) * (Nat.log2 n + 1) ^ d) r' := by sorry
Source
Torben Hagerup, Sorting and Searching on the Word RAM, STACS 1998, pp. 366-398, https://doi.org/10.1007/BFb0028575; Josh Alman and Virginia Vassilevska Williams, Truly Subquadratic 3SUM and Truly Subcubic APSP via Triangles in Sparse Lopsided Graphs, 2026, https://arxiv.org/abs/2610.06783; mission 'Two WordRAM models: equivalence with polylogarithmic overhead', fine-grained equivalence.

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