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Absolute Square


AbsSquareReal
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AbsSquareReImAbs
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The absolute square of a complex number z, also known as the squared norm, is defined as

 |z|^2=zz^_,
(1)

where z^_ denotes the complex conjugate of z and |z| is the complex modulus.

If the complex number is written z=x+iy, with x and y real, then the absolute square can be written

 |x+iy|^2=x^2+y^2.
(2)

If z=x+0i is a real number, then (1) simplifies to

 |z|^2=x^2.
(3)

An absolute square can be computed in terms of x and y using the Wolfram Language command ComplexExpand[Abs[z]^2, TargetFunctions -> {Conjugate}].

An important identity involving the absolute square is given by

|a+/-be^(-idelta)|^2=(a+/-be^(-idelta))(a+/-be^(idelta))
(4)
=a^2+b^2+/-ab(e^(idelta)+e^(-idelta))
(5)
=a^2+b^2+/-2abcosdelta.
(6)

If a=1, then (6) becomes

|1+/-be^(-idelta)|^2=1+b^2+/-2bcosdelta
(7)
=(1+/-b)^2∓4bsin^2(1/2delta).
(8)

If a=1, and b=1, then

 |1-e^(-idelta)|^2=4sin^2(1/2delta).
(9)

Finally,

|e^(iphi_1)+e^(iphi_2)|^2=(e^(iphi_1)+e^(iphi_2))(e^(-iphi_1)+e^(-iphi_2))
(10)
=2[1+cos(phi_2-phi_1)]
(11)
=4cos^2[1/2(phi_2-phi_1)].
(12)

See also

Complex Argument, Complex Modulus, Complex Number, Imaginary Part, Real Part, Sign

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Cite this as:

Weisstein, Eric W. "Absolute Square." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/AbsoluteSquare.html

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