recurrence relation calculation comparison error

I dont get the idea how to calculate the recurrsive relation .I started with following code but got stuck in the for loop

syms x a B

U=sym(zeros(1));

U(1)=a;

alpha=10;

g=0.01;

B=sym(zeros(1));

for k=1:10

B=0;

for m=1:k

B=B+U(m)*(k-m+1)*(k-m+2)*U(k-m+2)

end

k*(k+1)*U(k+2)+alpha*B=g*U(k)

end

The problem is attached

[Torsten]

Solve for U(k+2) using paper and pencil.

Then you get an explicit recursion.