Could anyone help me out with this equation please?
How to prove it? Any ideas?
You have a summation that starts at n=2.
Rewrite the summation of the second term by replacing n-1 by for instance k.
And rewrite the summation of the third term by replacing n+1 by m.
Afterward add or subtract whatever you need to, to make the summations start at 1.
Then you can combine them again.