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Cosine function

Function

definiendum cos:CC
definiendum cos(z):=k=0(1)k(2k)!z2n

Discussion

θR

cos(θ)=12(eiθ+eiθ)

i.e. if ζ:=eiθ, then ζ+¯ζ=2cos(θ).

Parents

Context

Infinite sum of complex numbers, Factorial function

Exponential function, Sine function