Note that the rank of B is 2.

Then the rank of B 2 is 1, and the rank of B 3 is 0.

That is, b n greater than cubic is actually a zero matrix.

So substitute it into the n expansion.

Except e n, ne (n- 1) b, n (n- 1)/2e (n-2) b? , everything else is 0.

So the result is e n+ne (n-1) b+n (n-1)/2e (n-2) b?