Complex Analysis - looking for a certain book (screenshot included)

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I hope this kind of question is tolerated and not against any rule. After searching for a problem i stumpled upon this question

Proving all conformal mappings from the upper half-plane to the unit disc take a certain form

and i am looking for the book from which the given screenshot has been taken of:

enter image description here

Does someone happen to recognize what book that could be? I would appreciate any hint.

Thank you very much.