2K(k)π
=
1+(12)2k2+(1⋅32⋅4)2k4+(1⋅3⋅524⋅3)2k6+(1⋅3⋅5⋅727⋅3)2k8+⋯+[(2n−1)!!2nn!]2k2n+⋯
Gradshteyn & Ryzhik (1965), §8.113.1