∫ − ∞ ∞ ( | s ( t ) | ) 2 d t {\displaystyle \int _{-\infty }^{\infty }(|s(t)|)^{2}dt} in time transforms to ∫ − ∞ ∞ ( | S ( f ) | ) 2 d f {\displaystyle \int _{-\infty }^{\infty }(|S(f)|)^{2}df} in frequency
in time transforms to 
in frequency