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I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

 

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to a poset $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$

I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to a poset $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$

I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to a poset $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$

I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to a poset $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$

I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to an ordered set $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$

I think that the source is Richard Dedekind's Was sind und was sollen die Zahlen ? (Vieweg: Braunschweig, 1888).

I quote from the English translation : THE NATURE AND MEANING OF NUMBERS, (The Open Court Publishing Co., 1901) :

98. Definition [page 37]. If $n$ is any number, then will we denote by $Z_n$ the system [set] of all numbers that are not greater than $n$ [...].

106. Theorem [page 38]. If $m < n$, then is $Z_m$ proper part of $Z_n$ and conversely.

I've found it through :

• Gregory Moore, Zermelo's Axiom of Choice : Its Origin Development and Influence (1982), page 26.

We can easily generalise it to a poset $M$ whatever, defining the mapping :

$\pi : a \to M_a$, for any $a \in M.$

where $M_a = \{ x \in M : x \le a \}$