feat(ErdosProblems): 726

[danielchin]

Closes #917

Formalizes https://www.erdosproblems.com/726

Assisted by Gemini/Antigravity.

[github-actions]

👋 This is an automated welcome message. 🤖
Thanks for the contributions!

A few friendly reminders while the review gets started:

• Please take a look at the style guidelines,
  especially the conventions for references, categories, AMS tags, and answer(sorry).
• You can manage some PR labels by leaving a comment with +label-name or -label-name; for example, +awaiting-author or -awaiting-author.
• This repository is mainly for formalised statements. Proofs longer than about 25-50 lines are usually out of scope; longer proofs are welcome to be included/linked via the formal_proof mechanism.

Thanks again for helping improve Formal Conjectures.

[mo271]

Thanks, LGTM, just one minor nit

[danielchin]

-awaiting-author

[mo271]

Thanks again!