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Imaginary part
From OeisWiki
ℑ (z) 
z = r e i θ = r cos θ + i r sin θ = a + i b = ℜ (z) + i ℑ (z) 

ℑ (z) := b =
= r sin θ,z − z̅ 2
z̅ 
z 
For example,
ℑ (3 + √ − 5 ) = √ 5 
A few sequences in the OEIS involve imaginary parts, such as for example A013629, floor of imaginary parts of nontrivial zeros of Riemann zeta function.
Compare real part.