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In this section we examine the case p=7 and obtain an iteration
that converges to
to
7-th order
. The proofs of some of the results of
this section have been omitted. They will appear in a later version of this
organic paper.
Our idea is to mimic
the cubic iteration of Section 6. The quadratic form in the cubic
case is
, which has
discriminant
-3 and
class number
.
The case p=7 is the next case to consider since the next odd, negative
discriminant
with
class number
1 is -7. The corresponding quadratic form is
. See [9] for a treatment of the classical theory
of binary quadratic forms. We have

where

Also,

Equation (7.1) with (7.2), and (7.4)
are contained in Entry 5(i) of Chapter 21 of Ramanujan's second
notebook [13], [1, p. 467,].
Equation (7.3) is analogous to (6.8) but
does not appear in Ramanujan's notebooks. We expect a result like (7.3)
to hold since the
class number
. A result of Legendre's
[11, Equation (1), p. 97,] gives an equivalent formulation
in terms of
L-series:

We define the following functions (analogous to the cubic case):

and for j=1,2,3

In a later version of this paper we will examine these functions
under the action of the
congruence subgroup
,
.
The proof of the following is analogous to the cubic case
[7, Lemma 2.1, p. 36,]:

From (7.9)--(7.12) we easily find that

For j=1,2,3 we define

and

From (3.14), (7.1) we have

Throughout this section q and r are related by

If we write the
as functions of r then by using the multidimensional
analogue [0]
of (3.3) it can be shown that

for j=1, 2, 3.
By (7.14), (7.19) we have

As noted in Section 4.3 the main problem
of constructing a 7-th order iteration is to find a relationship
between
and
. In view of (7.22)
and (7.23), we would like to find a relationship
between
and
. Instead we get each of the
in terms
of
,
,
. This is achieved by solving a certain
cubic and taking 7-th roots.
We now give some details of this construction.
Let

Now we define the following functions:

Then by using the theory of
modular functions
it can be shown that

Also,

Our functions
and
are related. We find that

Hence
are the roots of the following
cubic equation

Now observe that

Similarly we find that

Thus after solving a certain cubic, and taking 7-th roots
we may obtain each of the
in terms of
,
, and
.
From (3.18) we have

Finally, to construct an septic iteration, we need some initial
values. Two obvious candidates are
,
. From (3.10) we have

We need to find the values of the
for r=1. Fortunately, by (7.21),
these coincide with the values of the
. So when r=1,
and
. Thus we can in this case solve equations (7.27) and
(7.33) to obtain

Thus via (7.34) we find that
,
,
are roots of the cubic

Then after solving this cubic we can find the initial values
using (7.36). To obtain the septic iteration we need only
consider
in our identities and equations to obtain the following
theorem.
Theorem 7.1
Define sequences
,
,
,
and
by

where

and
are the roots of the cubic equation


where
are the roots of the cubic

where

and where



where

and

Then
converges septically to
.

Contents
Next: Nonic Iterations
Up: Approximations to via the
Previous: The Cubic Iteration