Finding number of integers not satisfying an expression

Hi,

For any a, b >= 0, how can one find the number of positive integers which *cannot* be expressed as ax + by where gcd(x,y) = 1 and x,y > 1

E.g for x,y = 3,4 the number of such integers is 3

I think that extended euledian algorithm can be used here but am not sure how.

Thanks.

Re: Finding number of integers not satisfying an expression

Given coprime positive integers x, y, the set of positive integers that cannot be represented as ax + by for some nonnegative integers a, b is called the set of gaps of a numerical semigroup with two generators x and y. The number of elements in the set of gaps is called the genus of the semigroup. In the case of two generators x and y, the genus equals (x − 1)(y − 1) / 2.

Re: Finding number of integers not satisfying an expression