\def\mod{\mathbin{\rm mod}}
\def\Near{\mathord{\rm Near}}
\def\rem{\mathord{\rm rem}}
\def\quo{\mathord{\rm quo}}

%119
\noindent{\bf 9.4\quad EXPONENTIATION}

\medskip\noindent
In this section we show that the function $m^n$ is diophantine, thus completing
the proof of the main theorem.  We have to begin by considering the
behaviour of the solutions to some difference equations.

Let $a$ be an integer greater than $1$.  Define $x_n$ and $y_n$ by
$$\matrix{x_{n+1} = 2ax_n-x_{n-1},& x_0 = 1,& x_1 = a,\cr\noalign{\smallskip}
y_{n+1} = 2ay_n - y_{n-1},& y_0 = 1,& y_1 = 1.}$$
When the parameter $a$ needs consideration we refer to these as $x_n(a)$ and
$y_n(a)$.

%120
{\it Let $d = a^2-1$\/}; then $\sqrt d$ is irrational.  It follows that if $m+n\sqrt d = u+v\sqrt d$
then $m = u$ and $n = v$, and so we also have $m-n\sqrt d = u-v\sqrt d$.

\medskip\noindent(I)\quad
It is easy to see, by induction, that $x_n\pm y_n\sqrt d = (a\pm\sqrt d)^n$.  Since
$(a+\sqrt d)(a-\sqrt d) = 1$, we also have that $x_n-y_n\sqrt d = (a+\sqrt d)^{-n}$.  Hence
$x_n^2-dy_n^2 = 1$.

\medskip\noindent(II)\quad
Conversely, we show that any solution of $x^2-dy^2 = 1$ with $x$ and $y$ in ${\bf N}$
must satisfy $x = x_n$ and $y = y_n$ for some $n$.  Observe first that there must be
some $n$ with $(a+\sqrt d)^n\le x+y\sqrt d < (a+\sqrt d)^{n+1}$.  Let $u+v\sqrt d$ be
$(x+y\sqrt d)(a-\sqrt d)^n$, so both $u$ and $v$ are integers.  We also have
$u-v\sqrt d = (x-y\sqrt d)(a+\sqrt d)^n$, and so $u^2-dv^2 = x^2-dy^2 = 1$.

Since $(a-\sqrt d)^n = (a+\sqrt d)^{-n}$, we have $1\le u+v\sqrt d < a+\sqrt d$.  Taking
the inverse of these inequalities and changing sign, we find that $-1\le
-u+v\sqrt d < -a+\sqrt d$.  Adding these to the original inequalities then gives
$0\le 2v\sqrt d < 2\sqrt d$.  Hence $v = 0$, and so $u = 1$, since $u^2-dv^2 = 1$.  It follows
that $x+y\sqrt d = (a+\sqrt d)^n$, so $x = x_n$ and $y = y_n$, as required.

\medskip\noindent(III)\quad
It follows easily from the equalities $x_n\pm y_n\sqrt d = (a\pm\sqrt d)^n =
(a+\sqrt d)^{\pm n}$ that $x_{m\pm n}+y_{m\pm n}\sqrt d = (x_m+y_m\sqrt d)(x_n+y_n\sqrt d)$, and hence
that we have the addition formulas $x_{m\pm n} = x_m x_n\pm dy_m y_n$, and
$y_{m\pm n} = x_n y_m\pm x_m y_n$.  In particular $x_{n\pm 1} = ax_m\pm dy_m$, $y_{m\pm 1} = ay_m\pm x_m$.  It
follows that $y_{m+1} > y_m$, and hence that $y_m\ge m$ for all $m$.

\medskip\noindent(IV)\quad
We denote the greatest common divisor of two integers $r$ and $s$ by
$(r,s)$.  Since $x_n^2-dy_n^2 = 1$, we see that $(x_n,y_n) = 1$.  By induction on $k$, using
the addition formulas, we see that $y_n|y_{nk}$ for all $n$ and $k$, where $r|s$ means that
$r$ divides $s$.

\medskip\noindent(V)\quad
We can now deduce the $y_n|y_t$ iff $n|t$.  The previous remark gives the
result if $n|t$.  Assume that $y_n|y_t$ and that $n$ does not divide $t$.  Write $t$ as $nq+r$,
where $0 < r < n$.  Since $y_t = x_r y_{nq}+x_{nq}y_r$, and we already know that $y_n$
divides $y_nq$, we see that $y_n|x_{nq}y_r$.  As $(y_{nq},x_{nq}) = 1$, and $y_n|y_{nq}$, it follows that
$y_n|y_r$.  But we know that $y_r < y_n$, a contradiction.

\medskip\noindent(VI)\quad
We next show that $y_{nk}\equiv kx_n^{k-1}y_n\mod y_n^3$.  To see this, we write
$x_{nk}+y_{nk}\sqrt d$ as $(x_n+y_n\sqrt d)^k$.  Expanding this by the binomial theorem, and
equating the terms involving $\sqrt d$ gives the result.

\medskip\noindent(VII)\quad
It follows immediately that if $k = y_n$ then $y_n^2|y_{nk}$.  As a partial
converse to this we show that $y_n|t$ if $y_n^2|y_t$.  We already know that $n|t$, since
$y_n|y_t$.  Write $t$ as $nk$.  Then we know that $y_n^2$ must divide $kx_n^{k-1}y_n$, so that $y_n$
divides $kx_n^{k-1}$.  Since $(y_n,x_n) = 1$, this tells us that $y_n$ divides $k$.

\medskip\noindent(VIII)\quad
From the difference equation, easy inductions (the initial cases
when $n$ is $0$ or $1$ being obvious) give the following results.  $y_n\equiv n\mod 2$, and
$y_n\equiv n\mod(a-1)$.  Also if $a\equiv b\mod c$ we find that, mod $c$, $x_n(a)\equiv x_n(b)$ and
$y_n(a)\equiv y_n(b)$.

%121
\medskip\noindent(IX)\quad
We now show that $x_{2n\pm j}\equiv -x_j\mod x_n$, and hence $x_{4n\pm j}\equiv x_j\mod x_n$.
For the  addition formula tells us that $x_{2n\pm j}\equiv x_n x_{n\pm j}+dy_n y_{n\pm j}\equiv
dy_n(y_n x_j\pm x_n y_j)\equiv dy_n^2x_j\equiv(x_n^2-1)x_j\equiv -x_j$.

\medskip\noindent(X)\quad
The most difficult of our results, is the following.  Let $x_i\equiv x_j\mod x_n$,
with $i\le j\le 2n$ and $n > 0$.  Then $i = j$, unless $a = 2$, $n = 1$, $i = 0$, and $j = 2$.

We first suppose that $x_n$ is odd, and let $q = (x_n-1)/2$.  Then no two of the
numbers $-q,\ldots,q$ are congruent mod $x_n$, and every integer is congruent to
one of them.  Now we know that $1 = x_0 < x_1 < \ldots < x_{n-1}$, and that
$x_{n-1}\le x_n/a\le x_n/2$.  In particular $x_{n-1}\le q$.  Also we know that the numbers
$x_{n+1},\ldots,x_{2n}$ are congruent mod $x_n$ to $-x_{n-1},\ldots,-x_0$, by the previous
result.  Hence the numbers $x_0,\ldots,x_{2n}$ are mutually incongruent mod $x_n$.

When $x_n$ is even, we let $q$ be $x_n/2$.  In this case $-q\equiv q \mod x_n$.  The result
will follow as before unless $x_{n-1} = q$; this possiblity would give $x_{n+1} = -q$.
However this case requires $x_n = 2x_{n-1}$, and, as we know that $x_n = ax_{n-1}+
dy_{n-1}$, we must have $a = 2$ and $y_{n-1} = 0$, so that $n = 1$, as needed.

The previous result can be extended to show that if $x_j\equiv d_i\mod x_n$, with
$0 < i\le n$ and $0\le j < 4n$, then either $j = i$ or $j = 4n-i$.  For if $j\le 2n$, we get
$i = j$ unless we are in the exceptional case.  Since $i$ is not $0$, we would then
have $i = 2$, $j = 0$, and $n = 1$, contradicting $i\le n$, so this case cannot happen.
If $j > 2n$ define $k$ as $4n-j$.  Then $k > 0$, and $x_k = x_j = x_i\mod x_n$.  As $0 < k < 2n$,
we cannot be in the exceptional case, and so must have $k = i$.

It follows at once that if $0 < i\le n$ and $x_i\equiv x_j\mod x_n$ then $j\equiv\pm i\mod 4n$.
For we need only write $j$ as $4nq+k$, where $0\le k < 4n$ and use the previous
result and the fact that $x_{4n+r}\equiv x_r$ for all $r$.

We are now in a position to prove the main results.

\bigskip\noindent{\bf Proposition 9.9}\quad
{\it The function $y_k(a)$ is diophantine.}

\medskip\noindent{\bf Proof}\quad
We show that, provided $k$ is non-zero (the general case can be
obtained from this, but is not needed) $y = y_k(a)$ iff $a > 1$ (else $y_k(a)$ is not
defined) and there are positive integers $x, u, v, s, t, b, r, p ,q, c$, and $d$
satisfying the following eight equations.
$$\eqalignno{x^2-(a^2-1)y^2 &= 1&(1)\cr
u^2-(a^2-1)v^2 &= 1&(2)\cr
s^2-(b^2-1)t^2 &= 1&(3)\cr
v &= ry^2&(4)\cr
b &= 1+4py = a+qu&(5)\cr
s &= x+cu&(6)\cr
t &= k+4(d-1)y&(7)\cr
y&\ge k.&(8)}$$

%122
First suppose the equations have a solution.  As the variables are not
zero, there will, by (II), be a positive $i, j$, and $n$ such that $x = x_i(a)$, $y = y_i(a)$,
$u = x_n(a)$, $v = y_n(a)$, $s = x_j(b)$, and $t = y_i(b)$.  By (4), $y\le v$, and so $i\le n$.
Form (5), $b\equiv a\mod x_n(a)$, from which we know, by (VII), that $x_j(b)\equiv x_j(a)
\mod x_n(a)$.  From (t) we have $x_j(b)\equiv x_i(a)\mod x_n(a)$.  Hence we have
$x_j(a)\equiv x_i(a)\mod x_n(a)$, so (X) tells us that $j\equiv\pm i\mod 4n$.

Equation (4) tells us that $y_i(a)^2|y_n(a)$, from which we know that $y_i(a)|n$.
Hence $j\equiv\pm i\mod 4y_i(a)$.

We know that $y_j(b)\equiv j\mod(b-1)$.  By (5), $b\equiv 1\mod 4y_i(a)$, so that
$y_j(b)\equiv j\mod 4y_i(a)$.  By (7), we have $y_j(b)\equiv k\mod 4y_i(a)$.  Consequently
$k\equiv\pm i\mod 4y_i(a)$.

However, we know that $i\le y_i(a)$, and condition (8) tells us that
$k\le y_i(a)$.  The congruence then requires that $i = k$.  As $i$ is defined to satisfy
$y = y_i(a)$, we have, as required, $y = y_k(a)$.

Conversely, suppose that $y = y_k(a)$.  Set $x$ to be $x_k(a)$, so that (1) holds.
Let $m = 2ky_k(a)$, and define $u = x_m(a)$, $v = y_m(a)$.  Then (2) holds.  We find
that $y^2|v$, since we have that $y^2|y_{m/2}(a)$, and the latter divides $y_m(a)$.  Hence
we can find $r$ satisfying (4).  Since $m$ is even, we know from (VIII) that $v$ is
even, and hence $u$ is odd.

As $u$ is odd and $(u,v) = 1$, adn $y$ divides $v$, we find that $(u,4y) = 1$.  By the
Chinese Remainder Theorem (Theorem 9.11 below) there is a non-negative
$b_0$ satisfying $b_0\equiv 1\mod 4y$ and $b_0\equiv a\mod u$.  Adding a suitable multiple of
$4uy$ to $b_0$, we can find positive $b,p$ and $q$ satisfying (5).  Then (3) is satisfied
by putting $s$ and $t$ to be $x_k(b)$ and $y_k(b)$.

Since $b > a$, we have $s > x$, as $s = x_k(b)$ and $x = x_k(a)$.  It also follows,
since $b\equiv a\mod u$, that $s\equiv x\mod u$.  Hence we can choose $c$ to satisfy (6).

Since $t$ is $y_k(b)$, we know that $t\ge k$ and $t\equiv k\mod(b-1)$.  By (5) we have
that $t\equiv k\mod 4y$, so that (7) can be satisfied.  Finally (8) holds, since
$y = y_k(a)$. $\bullet$

\bigskip\noindent{\bf Theorem 9.10}\quad
{\it The function $a^n$ is diophantine.}

\medskip\noindent{\bf Proof}\quad
It is most convenient to assume that $n > 0$ and $a > 1$.  We do not need
the general case; it can be obtained by a simple modification.

We define $\Near(u,v)$ to be the nearest integer to $v/u$.  The function $\Near$
is diophantine, since $\Near(u,v)\break= w$ iff $(2\rem(u,v)\le u \land w = \quo(u,v)) \lor
(2\rem(u,v) > u \land w = \quo(u,v)+1)$.

We shall show that $a^n = Near(y_{n+1}(k),y_{n+1}(ka))$ provided $k$ is large
enough.  By the previous proposition, $a^n$ will be diophantine if the require%-
ment that $k$ is large enough can be expressed as a diophantine condition.

It is ease to check, by induction, that, for any $b > 1$ and any $n$, $(2b-1)^n
\le y_{n+1}(b)\le (2b)^n$.  Apply this for $b = ka$ and for $b = k$, and divide the
inequalities.  We find that $y_{n+1}(ka)/y_{n+1}(k)$ lies between $(2ka-1)^n/(2k)^n =
a^n(1-1/2ka)^n$ and $(2ka)^n/(2k-1)^n = a^n(1-1/2k)^{-n}$.  Hence
$Near(y_{n+1}(k),y_{n+1}(ka)) = a^n$ if $(1-1/2ka)^n > 1-1/2a^n$ and
$(1-1/2k)^-n < 1+1/2a^n$.

%123
Since $y_{n+1}(a)\ge (2a-1)^n > a^n$, it is enough to require that $(1-1/2ka)^n
> 1-1/2y_{n+1}(a)$ and $(1-1/2k)^{-n} < 1+1/2y_{n+1}(a)$, and the latter con%-
dition can be written as $(1-1/2k)^n > (1+1/2y_{n+1}(a))^{-1}$.

Now $(1-x)^n > 1-nx$ for any real $x$ between $0$ and $1$ (because $(1-x)^n+
nx$ ahs a positive derivative).  Hence the required conditions hold provided
that $1-n/2ka > 1-1/2y_{n+1}(a)$ and $1-n/2k > (1+1/2y_{n+1}(a))^{-1}$.  These
two conditions,  when cleared of fractions, are diophantine conditions, since
$y_{n+1}(a)$ is diophantine. $\bullet$

\bye
