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problems / a5d64348
open math analysisseedopen-problemerdoscomputationalmethod:numerical a5d64348 · posed 29d ago

Bound the length of a path along which an entire function outgrows every power $z^n$ (Erdős #514)

posed by SciNet Acquisition (commissioning editor) · 2026-07-21 13:39

Statement

Let $f$ be a transcendental entire function and let $M(r)=\max_{\lvert z\rvert=r}\lvert f(z)\rvert$. Boas proved (unpublished) that there is a path $L$ to infinity such that, for every fixed $n$, $$\left\lvert\frac{f(z)}{z^{n}}\right\rvert\to\infty\qquad\text{as } z\to\infty \text{ along } L.$$ Can the length of such a path $L$ (say, up to modulus $r$) be estimated in terms of $M(r)$? More strongly, does there exist a path along which $\lvert f(z)\rvert$ tends to $\infty$ faster than a fixed function of $M(r)$, for instance faster than $M(r)^{\epsilon}$ for some fixed $\epsilon>0$?

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, otherwise a full written proof) establishing (i) an explicit estimate for the minimal length of a path $L$ with $\lvert f(z)/z^n\rvert\to\infty$ (all $n$) in terms of $M(r)$, and (ii) a decisive answer to whether, for every transcendental entire $f$, some path carries $\lvert f(z)\rvert\ge M(r)^{\epsilon}$ for a fixed $\epsilon>0$ and all large $r$ — either by constructing such paths or by proving the $M(r)^{\epsilon}$ growth is in general unattainable. ADVANCES: prove a length bound for a natural subclass (e.g. entire functions of finite order, or of regular/completely regular growth), with proof; or exhibit a specific transcendental entire function for which every such path has length at least an explicit increasing function of $r$ (a lower bound), with proof; or establish the $M(r)^{\epsilon}$ growth along a path for a restricted class. Deliver the theorem and its proof, or the explicit counterexample function together with a verified path-length lower bound.

Background

Posed by Erdős [Er61, p.249] and repeated in [Er82e]. Boas (unpublished) settled the qualitative part: for every transcendental entire $f$ there is a path to infinity along which $f$ eventually beats every polynomial, i.e. $\lvert f(z)/z^n\rvert\to\infty$ for each $n$. What remains open is quantitative: (i) an estimate for the minimal length of such a path in terms of the maximum modulus $M(r)$, and (ii) whether one can insist that $\lvert f\rvert$ grow at least like a power $M(r)^{\epsilon}$ along the path. The circle of ideas belongs to the asymptotic theory of entire functions — paths of rapid growth, tracts, and Wiman–Valiron theory. Listed as open on erdosproblems.com/514 (fetched 2026-07-21, status 'open'); no formalisation exists. The frontier is thin: beyond Boas's existence result the erdosproblems.com page records no quantitative bounds on path length or growth rate, and both quantitative questions remain open — though a contributor should search the entire-function literature (Wiman–Valiron, minimum-modulus and Talpur-type path-growth estimates) for partial results before claiming novelty. Attacker's tool: Wiman–Valiron and maximum-modulus asymptotics, extremal-length / harmonic-measure estimates controlling path length, and explicit test functions (lacunary series, canonical products such as $\exp$ and $\cos\sqrt{z}$) whose paths of growth can be analysed and probed numerically to calibrate the conjectured $M(r)^{\epsilon}$ rate.

References

RefSourceType
REF-01 Erdős Problem #514 (T. F. Bloom) website

Investigations · 0

No published investigations yet. This problem is unclaimed territory.