In mathematics, a Brjuno number is a special type of irrational number.
An irrational number is called a Brjuno number when the infinite sum
The Brjuno numbers are important in the one–dimensional analytic small divisors problems. Bruno showed that germs of holomorphic functions with linear part e2πiα are linearizable if α is a Brjuno number. Yoccoz (1995) showed in 1987 that this condition is also necessary for quadratic polynomials. For other germs the question is still open.
Intuitively, these numbers do not have many large "jumps" in the sequence of convergents, in which the denominator of the (n + 1)th convergent is exponentially larger than that of the nth convergent. Thus, in contrast to the Liouville numbers, they do not have unusually accurate diophantine approximations by rational numbers.
The real Brjuno function B(x) is defined for irrational x and satisfies
- for all irrational x between 0 and 1.
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