( x - y ) 2 U = ( x 4 + a 4 + y 4 + a 4 ) 2 - ( x 2 - y 2 ) 2 (x-y)^{2}U=\left(\sqrt{x^{4}+a^{4}}+\sqrt{y^{4}+a^{4}}\right)^{2}-(x^{2}-y^{2})^{2}