np.arrayで作った配列(array)とnp.matrixで作った行列(matrix)はほぼ同じに動くが，
x*yが，arrayだと要素同士の掛け算になるのに, matrixだと行列の掛け算になる．
なおどちらでもnp.dot(x,y)なら行列の掛け算になる．
array made by np.array and matrix made by np.matrix work in similar ways in most cases.
However, x*y becomes product of respective elements if x and y are arrays, but becomes product of matrix if x and y are matrix.
In regradless whether x and y are array or matrix, np.dot(x,y) gives product of matrix.