Add analysis and partial proof for Erdős Conjecture on Arithmetic Progressions (Problem 77)

[charlesen]

Added directory inprogress/77-Erdos-Conjecture-on-Arithmetic-Progressions containing:

• English and French PDF proofs of Lemma 2 for the conjecture on arithmetic progressions.
• LaTeX source files.
• English and French README files documenting the current status of the problem.
• A Lean 4 skeleton for autoformalization mapping the lemmas and axioms.

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PR created automatically by Jules for task 2201562748039553462 started by @charlesen

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