pub fn iterate(b: &Vec<f64>, a: &Vec<Vec<f64>>, prev: &Vec<f64>) -> (Vec<f64>, f64) {
    let n = b.len();
    let mut x = vec![0.0; n];
    let mut eps = vec![0.0; n];
    for i in 0..n {
        x[i] = b[i] + {
            let mut sum = 0.0;
            for j in 0..n {
                sum += a[i][j] * prev[j];
            }
            sum
        };
        eps[i] = (x[i]-prev[i]).abs();
    }

    //максимальная разность
    let mut max_eps = eps[0];
    for each in eps {
        if each > max_eps {
            max_eps = each;
        }
    }

    (x, max_eps)
}