Gergely Buday

2016-07-18 12:13:24 UTC

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Hi,Raw Message

I have proved a simple associativity rule and tried to prove commutativity of an addition function:

theory Add

imports Main

begin

datatype natural = Zero | Succ natural

primrec add :: "natural ⇒ natural ⇒ natural"

where

"add Zero m = m"

| "add (Succ n) m = Succ (add n m)"

lemma add_assoc: "⋀ k m n . add k (add m n) = add (add k m) n "

proof -

fix k m n

show "add k (add m n) = add (add k m) n"

proof (induct k arbitrary: m n)

fix m n

show "add Zero (add m n) = add (add Zero m) n"

proof (induct m arbitrary: n)

fix n

show "add Zero (add Zero n) = add (add Zero Zero) n" by simp

next

fix m n

assume "⋀n. add Zero (add m n) = add (add Zero m) n"

from this show "add Zero (add (Succ m) n) = add (add Zero (Succ m)) n" by simp

qed

next

fix k m n

assume "⋀m n. add k (add m n) = add (add k m) n"

from this show "add (Succ k) (add m n) = add (add (Succ k) m) n" by simp

qed

qed

text{*

Questions:

* is it usually better in an induction proof to name the other variables as arbitrary? Does this make the proof easier?

* I have created this proof quite mechanically, copying premise as an assumption and the conclusion as the show clause.

Is there a better proof strategy in general, and in particular for this proof?

*}

lemma add_commute: "⋀ m n. add m n = add n m"

proof -

fix m n

show "add m n = add n m"

proof (induct m arbitrary: n)

fix n

show "add Zero n = add n Zero"

proof (induct n)

show "add Zero Zero = add Zero Zero" by simp

next

fix n

assume "add Zero n = add n Zero"

from this show "add Zero (Succ n) = add (Succ n) Zero" by simp

qed

next

fix m n

assume indhyp: "⋀n. add m n = add n m"

from this show "add (Succ m) n = add n (Succ m)"

---

I am stuck here. There is no way of rewriting Succ m in the second parameter of Succ. What is the way out of the bottle?

Should I try another definition?

Should I use the proven theory of builtin natural numbers, prove an adequacy theorem of + and this add function and use the commutativity of + ?, or I can make it without this recourse?

- Gergely