(E=(Subscript[Σ, -∞]^∞)((\[LeftBracketingBar](Subscript[x, n])\[RightBracketingBar])^2)=(1/(2π))(∫_(-π)^π)((\[LeftBracketingBar]X(j ω)\[RightBracketingBar])^2)d ω.(20))