# x, y = numpy.ndarray of shape (n,)
# s_inv = numpy.ndarray of shape (n, n)
diff = x - y
d2 = diff.T.dot(s_inv).dot(diff)

diff <- x - y
d2 <- t(diff) %*% s_inv %*% diff

d = numpy.zeros((m,))
for i in range(m):
    diff = x[i,:] - mu
    d[i] = diff.T.dot(s_inv).dot(diff)