x1 = np.arange(27.0).reshape((3, 3, 3))
# x1 is (3,3,3)
x2 = x1.min(axis=(1,2))
# x2 is (3,)
(x1 - x2).shape
#Output: (3, 3, 3)

(x1 - x2).shape == x1.shape
#As expected: True

mat1 = np.random.rand(10,5,2,1)
# mat1 is (10,5,2,1)
mat2 = mat1.min(axis = (1,2,3))
# mat2 is (10,)
(mat1 - mat2).shape == mat1.shape
# Should be True, but
#Output: False