Sunday, March 29, 2009

Polynomials with Function Composition

This morning, reading in Computational Category Theory, by Rydeheard et al, I came across this little piece of ML (it's pretty early in the book. It takes me about 45 minutes to read a page, sheesh):

fun poly_eval(f) = f(f(3)) + f(3) + 3

And I got to thinking about how this looks like a linear combination of function compositions. I just finished Linear Algebra so everything looks linear. One of the things I've been trying to wrap my head around is how to express function composition in a linear manner, and while this posting won't address that directly, it may help me down that road.

The question today is how to express polynomials as composition, as in the code above.

Given a standard form for a degree two polynomial: $f(x)=a_1x^2+a_2x+a_3$, let's replace every occurrence of $x$ in the polynomial with a function that multiplies $x$ by its argument. Composing this function with itself would multiply the value of $x$ by something multiplied by the value of $x$.

Sort of like this:

let $f=\lambda a. ax$ where $a$ is the coefficient of the polynomial term. Here $x$ is a free variable and if we provide a coefficient $c$, the result is $cx$. If we compose these functions we can expand them and see what happens.
$\begin{eqnarray} f(f) & = & f (\lambda a. ax)\\& = & (\lambda b. bx)(\lambda\\\end{eqnarray}$

now apply a constant $c$ as our coefficient, and evaluate
$\begin{eqnarray} f(f c) & = & (\lambda b. bx)(\lambda c\\& = & (\lambda b.bx)(cx)\\& = & (cx)x\\& = & cx^2\end{eqnarray}$

and there we have our first term in our polynomial (I'm not up on $\lambda$-notation and such stuff, so please, someone correct me if I've made a mistake in the above).

We can express this in code (thanks to dolio on #haskell) by adopting the convention that $f$ will accept a function and a value where the value will become our free variable (using partial application) and the function will either be other $f$'s (composition) or $const$ $c$, the coefficient.

f::(Num a) => (a -> a) -> a -> a
f g x = g x * x

and these can be composed as many times as we like to get powers of $x$.

*Main> let two_x2 = f(f(const 2))
*Main> :t two_x2
two_x2 :: Integer -> Integer
*Main> two_x2 5

Unfortunately, this forces us to declare the coefficient in advance and then we're stuck with it. It would be nice to be able to decide the coefficient later and still have $x$ as a free variable.

We can do it like this:

x1 = \c -> f(const c)
x2 = \c -> f(f(const c))

Now both $x1$ and $x2$ (for $x$ and $x^2$) can be used to express a polynomial term with an arbitrary coefficient.

*Main> :t x2
x2 :: Integer -> Integer -> Integer
*Main> let five_x2 = x2 5
*Main> :t five_x2
five_x2 :: Integer -> Integer
*Main> five_x2 4
*Main> five_x2 2

Finally, it is simple to build arbitrary polynomials with a free variable

poly = \a1 a2 a3 x -> x2 a1 x + x1 a2 x + a3

where $a1, a2, a3$ are the coefficients. Using partial application we can create a specific polynomial with its coefficients and then apply it to any value for $x$ that we like.

*Main> let polyA = poly 1 2 3
*Main> :t polyA
polyA :: Integer -> Integer
*Main> polyA 5

Here, $polyA(x) = x^2 + 2x + 3$. There we go, arbitrary polynomial expression using function composition. I suppose this is pretty simple stuff, but I find it incredibly fascinating. I'm hoping this will have some application to my thoughts on linear transformations of functions with composition.

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