∑nr=1(∑r−1k=0nCrrCk 2k) is equal to
4n−3n
4n−3n+1
4n−3n−1
4n−3n+2
∑nr=1 nCr(∑nk=0rCk 2k−2n)∑nr=1nCr(3n−2n)=4n−3n