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)