Erdős Problem #196 — The question of whether or not monotone 4-term A.
The question of whether or not monotone 4-term A.P.'s must occur is currently completely open.
References
Additional references
Erdős, P. and Graham, R., Old and new problems and results in combinatorial number theory. Monographies de L'Enseignement Mathématique 28 (1980).
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