# GSoC Week 12

Last week I told you why `rs_series`

doesn’t work with negative or fractional
powers because of the constraints of a polynomial back-end and why we need to
modify `polys`

. The situation isn’t that hopeless actually. Let’s talk about
negative and fractional powers one by one.

### Negative Powers

The reason negative exponents work in `ring_series`

is because I modified
`PolyElement`

to allow so. In hind sight, it wasn’t the right decision and needs
to be replaced with something that doesn’t alter `polys`

. It is rather
surprising that I came across a possible solution so late (Now we know why good
documentation is so important).

I already knew that `polys`

allows us to create a `FractionField`

. A fraction
field over a domain `R`

consists of elements of the form `a/b`

where `a`

and `b`

belong to `R`

. In our case we are interested in the fraction field of polynomial
ring, i.e, fractions with polynomials as numerator and denominator. So `a/b`

is
not `a * b**(-1)`

but is `a / b`

, where a and b are polynomials.

What was new to me was that just like `ring`

, `polys`

also has sparse `field`

.
In effect, it allows us to create sparse rational functions without altering
anything.

I modified some functions in `ring_series`

to work with a rational function
field here, and it works quite well indeed.

```
In [42]: from sympy.polys.fields import *
In [43]: F, a, b = field('a, b', QQ)
In [44]: p = rs_sin(a + b, a, 3)/a**5
In [45]: p*a
Out[45]: (-3*a**2*b - 3*a*b**2 + 6*a - b**3 + 6*b)/(6*a**4)
```

Note that all these are field operations and I haven’t modified `field.py`

in
any way. Elegant!

But then again, having a field increases the complexity as we need to evaluate the numerator and denominator separately.

### Fractional Powers

Fractional powers are a much trickier case as there is no simple solution to it as
above. What we can do is optimise the option I had presented in my last post,
i.e, have each fractional power as a generator. But doing that opens up a
Pandora’s box. Simple things such as `sqrt(a)**2 == a`

do not hold true any
more. The current `rs_series`

treats `sqrt(a)`

as a constant if we are expanding
with respect to `a`

:

```
In [22]: rs_series(sin(a**QQ(1,2)), a**QQ(1,2),5)
Out[22]: -1/6*(sqrt(a))**3 + (sqrt(a))
In [23]: rs_series(sin(a**QQ(1,2)), a,5)
Out[23]: sin(sqrt(a))
```

So, if we indeed decide to tread this path, we would need to replace `a`

here
with `sqrt(a)**2`

. This really complicates the situation as we need to figure
out what to replace with. In any calculation the powers change multiple times
and each time we’ll need to figure out how to rewrite the series.

### Next Week

It is now mostly a design decision whether we want the `ring_series`

to be
confined within the `polys`

module. The `polys`

environment allows efficient
manipulations of laurent series (with `FracField`

), but I doubt we can achieve
the speed we want with puiseux series without modifying `polys`

. One possible
solution is to separate the modified parts of `polys`

along with `ring_series`

from `polys`

. We are using `polys`

only because it has the data structure that
we want. Separating them would allow us to simultaneously make use of its
back-end and not introduce unnecessary complexity in our representation.

Other than that, documentation is another priority now. I had planned to do it earlier too, but couldn’t. This week’s discovery has reminded me of its importance.

Cheers!

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