Π ( α 2 , k ) − K ( k ) = 1 3 α 2 R J ( 0 , k ′ 2 , 1 , 1 − α 2 ) \Pi\left(\alpha^{2},k\right)-K\left(k\right)=\tfrac{1}{3}\alpha^{2}R_{J}\left(0,{k^{\prime}}^{2},1,1-\alpha^{2}\right)