: <p><math>\operatorname{E}f = xy\ \operatorname{E}f_{x y} + x (\!| y |\!)\ \operatorname{E}f_{x (\!| y |\!)} + (\!| x |\!) y\ \operatorname{E}f_{(\!| x |\!) y} + (\!| x |\!)(\!| y |\!)\ \operatorname{E}f_{(\!| x |\!)(\!| y |\!)}.</math></p> | : <p><math>\operatorname{E}f = xy\ \operatorname{E}f_{x y} + x (\!| y |\!)\ \operatorname{E}f_{x (\!| y |\!)} + (\!| x |\!) y\ \operatorname{E}f_{(\!| x |\!) y} + (\!| x |\!)(\!| y |\!)\ \operatorname{E}f_{(\!| x |\!)(\!| y |\!)}.</math></p> |