There are two ways to formally introduce a function in DC Proof:
1. Simply define it without any justification.
EXAMPLE
To define addition on the natural natural numbers, you can introduce an axiom in a single line, something like:
1. ALL(a):ALL(b):[a in n & b in n => a+b in n]
& ALL(a):[a in n => a+0=a]
& ALL(a):ALL(b):[a in n & b in n => a+(b+1)=(a+b)+1
Axiom
Where n is the set of natural numbers.
2. Actually prove the existence of the required function, using previously introduced axiom(s) at beginning of your proof, the axioms of set theory (on the Sets menu) and the rules of logic (on the Logic menu). Could take hundreds of lines of formal proof. (Not for the faint of heart!)
EXAMPLE
Given Peano's Axioms for the natural numbers, prove:
EXIST(add):[ALL(a):ALL(b):[a in n & b in n => add(a,b) in n]
& ALL(a):[a in n => add(a,0)=a]
& ALL(a):ALL(b):[a in n & b in n => add(a,s(b))=s(add(a,b))]]
Where n is the set of natural numbers and s is the successor function from Peano's Axioms.
https://dcproof.com/ConstructAddFunction.htm (762 lines--I warned you!)
Then you may also want to prove that there is only one such function of the set of natural numbers:
ALL(add):ALL(add'):[ALL(a):ALL(b):[a in n & b in n => add(a,b) in n]
& ALL(a):[a in n => add(a,0)=a]
& ALL(a):ALL(b):[a in n & b in n => add(a,s(b))=s(add(a,b))]
& ALL(a):ALL(b):[a in n & b in n => add'(a,b) in n]
& ALL(a):[a in n => add'(a,0)=a]
& ALL(a):ALL(b):[a in n & b in n => add'(a,s(b))=s(add'(a,b))]
=> ALL(a):ALL(b):[a in n & b in n => add(a,b)=add'(a,b)]]
https://dcproof.com/AdditionOnNUnique.htm (only another 84 lines)
Dan
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