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In mathematics, discrepancy theory describes the deviation of a situation from the state one would like it to be in. It is also called the theory of irregularities of distribution. This refers to the theme of classical discrepancy theory, namely distributing points in some space such that they are evenly distributed with respect to some (mostly geometrically defined) subsets. The discrepancy (irregularity) measures how far a given distribution deviates from an ideal one.
Discrepancy theory can be described as the study of inevitable irregularities of distributions, in measure-theoretic and combinatorial settings. Just as Ramsey theory elucidates the impossibility of total disorder, discrepancy theory studies the deviations from total uniformity.
The three permutations problem (Beck) – disproved by Newman and Nikolov.
Erdős discrepancy problem – Homogeneous arithmetic progressions. The problem was stated by Erdős, who offered $500 for the proof or disproof of the conjecture. A computer-assisted proof of a special case of the conjecture was published in February 2014. In September 2015, Terence Tao announced a proof of the conjecture.
^Joel Spencer (June 1985). "Six Standard Deviations Suffice". Transactions of the American Mathematical Society. Transactions of the American Mathematical Society, Vol. 289, No. 2. 289 (2): 679–706. JSTOR2000258. doi:10.2307/2000258.