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Description: Define proper
substitution. Remark 9.1 in [Megill] p. 447 (p.
15 of the
preprint). For our notation, we use to mean "the wff
that results when
is properly substituted for in the wff
." We
can also use in
place of the "free for"
side condition used in traditional predicate calculus; see, for example,
stdpc4 1729.
Our notation was introduced in Haskell B. Curry's Foundations of Mathematical Logic (1977), p. 316 and is frequently used in textbooks of lambda calculus and combinatory logic. This notation improves the common but ambiguous notation, " is the wff that results when is properly substituted for in ." For example, if the original is , then is , from which we obtain that is . So what exactly does mean? Curry's notation solves this problem. In most books, proper substitution has a somewhat complicated recursive definition with multiple cases based on the occurrences of free and bound variables in the wff. Instead, we use a single formula that is exactly equivalent and gives us a direct definition. We later prove that our definition has the properties we expect of proper substitution (see theorems sbequ 1792, sbcom2 1936 and sbid2v 1945). Note that our definition is valid even when and are replaced with the same variable, as sbid 1728 shows. We achieve this by having free in the first conjunct and bound in the second. We can also achieve this by using a dummy variable, as the alternate definition dfsb7 1940 shows (which some logicians may prefer because it doesn't mix free and bound variables). Another alternate definition which uses a dummy variable is dfsb7a 1943. When and are distinct, we can express proper substitution with the simpler expressions of sb5 1839 and sb6 1838. In classical logic, another possible definition is but we do not have an intuitionistic proof that this is equivalent. There are no restrictions on any of the variables, including what variables may occur in wff . (Contributed by NM, 5-Aug-1993.) |
Ref | Expression |
---|---|
df-sb |
Step | Hyp | Ref | Expression |
---|---|---|---|
1 | wph | . . 3 | |
2 | vx | . . 3 | |
3 | vy | . . 3 | |
4 | 1, 2, 3 | wsb 1716 | . 2 |
5 | 2, 3 | weq 1460 | . . . 4 |
6 | 5, 1 | wi 4 | . . 3 |
7 | 5, 1 | wa 103 | . . . 4 |
8 | 7, 2 | wex 1449 | . . 3 |
9 | 6, 8 | wa 103 | . 2 |
10 | 4, 9 | wb 104 | 1 |
Colors of variables: wff set class |
This definition is referenced by: sbimi 1718 sb1 1720 sb2 1721 sbequ1 1722 sbequ2 1723 drsb1 1751 spsbim 1795 sbequ8 1799 sbidm 1803 sb6 1838 hbsbv 1890 |
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