Метод простой итерации для СЛУ

def sort_diag_arr(a, b) :
    n = len(a)
    for i in range(n):
        ind, a[i:] = zip(*sorted(enumerate(a[i:]),
                             key=lambda x: -x[1][i]))
        b[i:] = [b[i+j] for j in ind]
    
    return a, b
 
 
A = [
[3, 5, -1],
[1, 3, -9],
[9, 1, 3]]
B = [9, -3, 15]
A, B = sort_diag_arr(A, B)
 
eps = 1e-5
x = []
n = len(A)
for j in range(n):
    k = A[j][j]
    for i in range(n):
        A[j][i] /= -k 
    A[j][j] = 0
    B[j] /= k 
 
# r - счетчик итераций
r = 0
x = B.copy()
tmp = sum(x) + 2*eps
while abs(sum(x)-tmp) > eps:
    tmp = sum(x)
    t = [0]*n
    for i in range(n):
        t[i] = sum(x[j]*A[i][j] for j in range(n)) + B[i]
    r += 1
    x = t.copy()
print(*(round(elem,3) for elem in x))
print(r)

def step(x, y, z):
    return ((15-y-3*z)/9,
            (9-3*x+z)/15,
            (3+x+3*y)/9)
 
print("Первая итерация")
it = (0, 0, 0)
next = step(*it)
print(*next)
 
print("Вторая итерация")
it = next
next = step(*it)
print(*next)
 
print("Третья итерация")
it = next
next = step(*it)
print(*next)
 
while max(abs(next[0]-it[0]), abs(next[1]-it[1]), abs(next[2]-it[2])) > 1e-5:
    it = next
    next = step(*it)
 
print("Решение")
print(*next)
 
print("Невязка")
print(3*next[0]+15*next[1]-next[2]-9)
print(next[0]+3*next[1]-9*next[2]+3)
print(9*next[0]+next[1]+3*next[2]-15)