Find the cardinality (denoted by Ak(m)) of the largest subset of {1, 2, ..., m} which contains no progression of k distinct terms. The elements of the forbidden progressions are not required to be consecutive. For example, A4(10) = 8, because {1, 2, 3, 5, 6, 8, 9, 10} has no arithmetic progressions of length 4, while all 9-element subsets of {1, 2, ..., 10} have one.
In 1936, Paul Erdős and Pál Turán posed a question related to this number[2] and Erdős set a $1000 prize for an answer to it. The prize was collected by Endre Szemerédi for a solution published in 1975, what has become known as Szemerédi's theorem.
As of 2020[update], the longest known arithmetic progression of primes has length 27:[4]
224584605939537911 + 81292139·23#·n, for n = 0 to 26. (23# = 223092870)
As of 2011, the longest known arithmetic progression of consecutive primes has length 10. It was found in 1998.[5][6] The progression starts with a 93-digit number
^H. Dubner; T. Forbes; N. Lygeros; M. Mizony; H. Nelson; P. Zimmermann, "Ten consecutive primes in arithmetic progression", Math. Comp. 71 (2002), 1323–1328.