In mathematics, Midy's theorem, named after French mathematician E. Midy, is a statement about the decimal expansion of fractions a/p where p is a prime and a/p has a repeating decimal expansion with an even period (sequence A028416 in the OEIS). If the period of the decimal representation of a/p is 2n, so that

${\displaystyle {\frac {a}{p}}=0.{\overline {a_{1}a_{2}a_{3}\dots a_{n}a_{n+1}\dots a_{2n}}}}$

then the digits in the second half of the repeating decimal period are the 9s complement of the corresponding digits in its first half. In other words,

${\displaystyle a_{i}+a_{i+n}=9}$

${\displaystyle a_{1}\dots a_{n}+a_{n+1}\dots a_{2n}=10^{n}-1.}$

For example,

${\displaystyle {\frac {1}{13}}=0.{\overline {076923}}{\text{ and }}076+923=999.}$

${\displaystyle {\frac {1}{17}}=0.{\overline {0588235294117647}}{\text{ and }}05882352+94117647=99999999.}$