 Solution

ID: past-exam-of-the-logic-and-set-theory-course-of-the-university-of-oxford/2011/a/3/b/i/solution

t

x_{i}

ϕ

if, at every free occurrence of

x_{i}

to be replaced, no enclosing quantifier binds a variable occurring in

t

. Equivalently, the substitution introduces no variable capture. Only free occurrences of

x_{i}

are replaced.

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