What is the size of the largest $A \subseteq {1, \dots, n}$ such that if $a \leq b \leq c \leq d \in A$ are such that $ab c d$ is a square then $a d = b c$ ?

Worked, still open.

number theory · solved · formalized (Lean) · 0 attempts

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vela registry pull vfr_37aec80d874a0239

vela reproduce examples/erdos-problems

evidence

unverified AI candidates (2)

gpt-erdos · GPT-5.2 Pro + Deep Research · unverified

### ✅ Answer (with an explicit extremal formula)

llm-hunter · gpt pro 5.2 · unverified

1 LLM attack(s) recorded (gpt pro 5.2); unverified.

AMS 11 · open (literature)

theorem erdos_888 : ∀ n, Nat.findGreatest (p n) n = (answer(sorry) : ℕ → ℕ) n

proved · paper

solved