src/HOL/Isar_Examples/Summation.thy
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(*  Title:      HOL/Isar_Examples/Summation.thy
Author:     Markus Wenzel
*)

header {* Summing natural numbers *}

theory Summation
imports Main
begin

text {* Subsequently, we prove some summation laws of natural numbers
(including odds, squares, and cubes).  These examples demonstrate
how plain natural deduction (including induction) may be combined
with calculational proof. *}

subsection {* Summation laws *}

text {* The sum of natural numbers $0 + \cdots + n$ equals $n \times (n + 1)/2$.  Avoiding formal reasoning about division we prove this
equation multiplied by $2$. *}

theorem sum_of_naturals:
"2 * (\<Sum>i::nat=0..n. i) = n * (n + 1)"
(is "?P n" is "?S n = _")
proof (induct n)
show "?P 0" by simp
next
fix n have "?S (n + 1) = ?S n + 2 * (n + 1)" by simp
also assume "?S n = n * (n + 1)"
also have "... + 2 * (n + 1) = (n + 1) * (n + 2)" by simp
finally show "?P (Suc n)" by simp
qed

text {* The above proof is a typical instance of mathematical
induction.  The main statement is viewed as some $\var{P} \ap n$
that is split by the induction method into base case $\var{P} \ap 0$, and step case $\var{P} \ap n \Impl \var{P} \ap (\idt{Suc} \ap n)$ for arbitrary $n$.

The step case is established by a short calculation in forward
manner.  Starting from the left-hand side $\var{S} \ap (n + 1)$ of
the thesis, the final result is achieved by transformations
involving basic arithmetic reasoning (using the Simplifier).  The
main point is where the induction hypothesis $\var{S} \ap n = n \times (n + 1)$ is introduced in order to replace a certain subterm.
So the transitivity'' rule involved here is actual
\emph{substitution}.  Also note how the occurrence of \dots'' in
the subsequent step documents the position where the right-hand side
of the hypothesis got filled in.

\medskip A further notable point here is integration of calculations
with plain natural deduction.  This works so well in Isar for two
reasons.
\begin{enumerate}

\item Facts involved in \isakeyword{also}~/ \isakeyword{finally}
calculational chains may be just anything.  There is nothing special
about \isakeyword{have}, so the natural deduction element
\isakeyword{assume} works just as well.

\item There are two \emph{separate} primitives for building natural
deduction contexts: \isakeyword{fix}~$x$ and
\isakeyword{assume}~$A$.  Thus it is possible to start reasoning
with some new arbitrary, but fixed'' elements before bringing in
the actual assumption.  In contrast, natural deduction is
occasionally formalized with basic context elements of the form
$x:A$ instead.

\end{enumerate}
*}

text {* \medskip We derive further summation laws for odds, squares,
and cubes as follows.  The basic technique of induction plus
calculation is the same as before. *}

theorem sum_of_odds:
"(\<Sum>i::nat=0..<n. 2 * i + 1) = n^Suc (Suc 0)"
(is "?P n" is "?S n = _")
proof (induct n)
show "?P 0" by simp
next
fix n have "?S (n + 1) = ?S n + 2 * n + 1" by simp
also assume "?S n = n^Suc (Suc 0)"
also have "... + 2 * n + 1 = (n + 1)^Suc (Suc 0)" by simp
finally show "?P (Suc n)" by simp
qed

text {* Subsequently we require some additional tweaking of Isabelle
built-in arithmetic simplifications, such as bringing in
distributivity by hand. *}

theorem sum_of_squares:
"6 * (\<Sum>i::nat=0..n. i^Suc (Suc 0)) = n * (n + 1) * (2 * n + 1)"
(is "?P n" is "?S n = _")
proof (induct n)
show "?P 0" by simp
next
fix n have "?S (n + 1) = ?S n + 6 * (n + 1)^Suc (Suc 0)"
by (simp add: distrib)
also assume "?S n = n * (n + 1) * (2 * n + 1)"
also have "... + 6 * (n + 1)^Suc (Suc 0) =
(n + 1) * (n + 2) * (2 * (n + 1) + 1)" by (simp add: distrib)
finally show "?P (Suc n)" by simp
qed

theorem sum_of_cubes:
"4 * (\<Sum>i::nat=0..n. i^3) = (n * (n + 1))^Suc (Suc 0)"
(is "?P n" is "?S n = _")
proof (induct n)
show "?P 0" by (simp add: power_eq_if)
next
fix n have "?S (n + 1) = ?S n + 4 * (n + 1)^3"
by (simp add: power_eq_if distrib)
also assume "?S n = (n * (n + 1))^Suc (Suc 0)"
also have "... + 4 * (n + 1)^3 = ((n + 1) * ((n + 1) + 1))^Suc (Suc 0)"
by (simp add: power_eq_if distrib)
finally show "?P (Suc n)" by simp
qed

text {* Comparing these examples with the tactic script version
@{file "~~/src/HOL/ex/NatSum.thy"}, we note an important difference
of how induction vs.\ simplification is
applied.  While \cite[\S10]{isabelle-ref} advises for these examples
that induction should not be applied until the goal is in the
simplest form'' this would be a very bad idea in our setting.

Simplification normalizes all arithmetic expressions involved,
producing huge intermediate goals.  With applying induction
afterwards, the Isar proof text would have to reflect the emerging
configuration by appropriate sub-proofs.  This would result in badly
structured, low-level technical reasoning, without any good idea of
the actual point.

\medskip As a general rule of good proof style, automatic methods
such as $\idt{simp}$ or $\idt{auto}$ should normally be never used
as initial proof methods, but only as terminal ones, solving certain
goals completely.  *}

end