What do we get if we take the unit circle x² + y² = 1 and complexify it? We get the 3D object (manifold) in 4D space given by the equation
z² + w² = 1
where z, w are complex numbers. Here's one way to see its #geometry. (A slowmo animation to follow.) #calculus
Here is a slow animation of how circles of various radii (centered at the origin) are transformed by the complexified circle function. #math#geometry#calculus
N.B., the reason we see two disconnected loops for the image of circles of small radii is one loop corresponds to the + and other to the - choices in the function u(r,θ). (And, yes, the function w is analytic.)
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For any subset A of a topological space X, there are no more than 14 sets that can be formed from A using the closure and complement operations. The complement of B is B' = X-B. Closure C(B) = smallest closed set containing B. (Neat problem from Kelley's General #Topology.) #math
There's a lemma that helps one show this. Letting C(A) denote the closure of A, and N(A) the complement of A, we can prove:
Lemma: for all sets A, one has the equation
CNC N CNC(A) = CNC(A).
The result follows from the semigroup with identity I generated by 2 elements N and C such that N² = I, C² = C, and CNCNCNC = CNC. Its 14 elements are:
I N C NC CN NCN CNC CNCN NCNC
NCNCN CNCNC CNCNCN NCNCNC NCNCNCN.