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Summary
DescriptionGudermannian function.png
English: The w:Gudermannian function relates the area of a w:circular sector to the area of a w:hyperbolic sector, via a common w:stereographic projection. If twice the area of the blue hyperbolic sector is ψ, then twice the area of the red circular sector is φ = gd ψ. Twice the area of the purple triangle is the stereographic projection s = tan 1/2φ = tanh 1/2ψ. The blue point has coordinates (cosh ψ, sinh ψ). The red point has coordinates (cos φ, sin φ). The purple point has coordinates (0, s).
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