The Hausdorff ordered pair definition

Question

I have found on the internet several versions of the formal definition of ordered pair, presented by Felix Hausdorff.

For example:

(a,b) = { {a, 1} , {b, 2} }

(a,b) = { {a, O}, {b,{O}} } (O is indicating the empty set.)

I would like to know what Hausdorff's definition was really and in what work it is possible to find it. I ask for help.

Answer 1

Felix Hausdorff, Grundzüge der Mengenlehre (1914).

See page 32:

$\{ \{ a, 1 \}, \{ b,2 \} \}$.

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For Norbert Wiener's contribution, equivalent in modern terms to:

$\{ \{ \{ a \}, \emptyset \}, \{ \{ b \} \} \}$,

see: Norbert Wiener, A simplification of the logic of relations (1914), into: Jean van Heijenoort (editor), From Frege to Gödel: A Source Book in Mathematical Logic, Harvard U.P. (1967), page 224-27.