\[\left(1-\tfrac{1}{2}k^{2}\right)\sum_{p=0}^{\infty}C_{2p+1}^{2}-{\tfrac{1}{2}k%
^{2}\left(\tfrac{1}{2}C_{1}^{2}+\sum_{p=0}^{\infty}C_{2p+1}C_{2p+3}\right)=1},\]