| Description: Equality of a class
variable and a class abstraction (also called a
class builder). Theorem 5.1 of [Quine] p.
34. This theorem shows the
relationship between expressions with class abstractions and expressions
with class variables. Note that abbib 2350 and its relatives are among
those useful for converting theorems with class variables to equivalent
theorems with wff variables, by first substituting a class abstraction
for each class variable.
Class variables can always be eliminated from a theorem to result in an
equivalent theorem with wff variables, and vice-versa. The idea is
roughly as follows. To convert a theorem with a wff variable 𝜑
(that has a free variable 𝑥) to a theorem with a class variable
𝐴, we substitute 𝑥 ∈ 𝐴 for 𝜑
throughout and simplify,
where 𝐴 is a new class variable not already
in the wff. An example
is the conversion of zfauscl 4223 to inex1 4237 (look at the instance of
zfauscl 4223 that occurs in the proof of inex1 4237). Conversely, to convert
a theorem with a class variable 𝐴 to one with 𝜑, we substitute
{𝑥
∣ 𝜑} for 𝐴
throughout and simplify, where 𝑥 and 𝜑
are new setvar and wff variables not already in the wff. For more
information on class variables, see Quine pp. 15-21 and/or Takeuti and
Zaring pp. 10-13.
(Contributed by NM, 26-May-1993.) (Proof shortened by Wolf Lammen,
12-Feb-2025.) |
