|Description: Define the value of a
also known as function
application. For example, (we prove this in cos0 12393
after we define cosine in df-cos 12315). Typically function is
defined using maps-to notation (see df-mpt 4053 and df-mpt2 5797), but this
is not required. For example,
Note that df-ov 5795 will define two-argument functions using
Although based on the idea
embodied by Definition 10.2 of [Quine] p.
65 (see args 5029), our
definition apparently does not appear in the literature. However, it is
quite convenient: it can be applied to any class and evaluates to the
empty set when it is not meaningful (as shown by ndmfv 5486 and fvprc 5455).
The left apostrophe notation originated with Peano and was adopted in
Definition *30.01 of [WhiteheadRussell] p. 235, Definition
[Quine] p. 68, and Definition 6.11 of [TakeutiZaring] p. 26. It means
the same thing as the more familiar notation for a
function's value at , i.e. " of ,"
context-dependent notational ambiguity. Alternate definitions are
dffv2 5526 and dffv3 6253. For other alternate definitions
(that fail to
evaluate to the empty set for proper classes), see fv2 5454,
fv3 5474, and
fv4 6254. Restricted equivalents that require to be a function are
shown in funfv 5520 and funfv2 5521. For the familiar definition of function
value in terms of ordered pair membership, see funopfvb 5500.
(Contributed by NM, 1-Aug-1994.)|