Let a,b,c in CC, and define f(x,y)=ae^(i(x+y))-b(e^(ix)+e^(iy))+c

allucinemsj

allucinemsj

Answered question

2022-08-09

Let a , b , c C , and define
f ( x , y ) = a e i ( x + y ) b ( e i x + e i y ) + c
for x , y [ π , π ]

Answer & Explanation

Olivia Petersen

Olivia Petersen

Beginner2022-08-10Added 16 answers

With z = e i x and w = e i y we have f ( x , y ) = a z w  b z  b w + c for z = b w  c a w  b . The linear fractional transformation g : w  b w  c a w  b preserves the unit circle iff g ( ζ ) g ( 1 / ζ ¯ ) ¯ = 1 for all complex ζ it is described here. That reduces to
b ζ  c a ζ  b = a ¯  b ¯ ζ b ¯  c ¯ ζ 
Assuming b  0, the condition is a = b c ¯ b ¯ 

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