( 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}