# Deux suites "aléatoires" (mash de clavier)
x = lambda n : [ 5, 1, 8, 1, 23, 1, 5, 9, 7, 41 ][n] if n < 10 and n >= 0 else 0
y = lambda n : [ 8, 4, 6, 7, 1, 2, 5 ][n] if n < 7 and n >= 0 else 0

# w_n(k), la convolution dans Z/nZ
def w(n, k):
    r = 0
    for i in range(n):
        r += x(i % n) * y((k - i) % n)
    return r

# v(n), la convolution tout court
def v(n):
    r = 0
    for i in range(n - 6, 10):
        r += x(i) * y(n - i)
    return r

print(w(20, 18)) # c'est bien 0 !