... rank n matrices can have rank less than n. For instance, for any matrix H, the sumH + (−1) · H has rank zero.A sum of rank n matrices can have rank greater than n. Here are rank one matrices that ... Algebra, by HefferonThree.IV.3.29 For any scalar r and square matrix H we have (rI)H = r( IH) = rH = r( HI) =(Hr)I = H(rI).There are no other such matrices; here is an argument for 2×2 matrices that ... entryof (F G)H is thisst=1 r k= 1fi ,k g k, tht,j=st=1 r k= 1(fi ,k g k, t)ht,j=st=1 r k= 1fi ,k (g k, tht,j)= r k= 1st=1fi ,k (g k, tht,j) = r k= 1fi ,k st=1g k, tht,j(the...