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#### Table of Contents

# Remez tutorial 3/5: changing variables for simpler polynomials

In the previous section, we introduced function *g(x)* and saw that `RemezSolver`

looked for a polynomial *P(x)* and the smallest error value *E* such that:

Where *g(x)* is a function used as a weight to minimise relative error.

We will now see that *g(x)* can be used for other purposes.

## Odd and even functions

An **odd function** is such that in all points, *f(-x) = -f(x)*. Similarly, an **even function** is such that *f(-x) = f(x)*.

An example of an odd function is *sin(x)*. An example of an even function is *cos(x)*.

A reasonable expectation is that approximating an **odd function** over an interval centred at zero will lead to an **odd polynomial**. But due to the way the Remez exchange algorithm works, this is not necessarily the case. We can help the algorithm a little.

## The example of sin(x)

Suppose we want to approximate *sin(x)* over [-π/2; π/2]:

We know *sin(x)* is an odd function, so instead we look for an **odd polynomial**. Odd polynomials are polynomials that only use odd powers of *x*, such as *x*, *x³*, *x⁵*…

We notice that an odd polynomial *P(x)* can always be rewritten as *xQ(x²)* where *Q* has the same coefficients. Our minimax constraint becomes:

Another observation is that replacing *x* with *-x* has no effect on the expression in the left. We can therefore restrain the range to [0; π/2]:

Now since *x* is always positive, we can at last perform our **change of variable**, replacing *x²* with *y* and *x* with *√y̅*, and changing the range accordingly:

This is almost something `RemezSolver`

can deal with, if only there wasn’t this *√y̅* in front of *Q(y)*. Fortunately we can simply divide everything by *√y̅*:

That’s it! We have our minimax equation, and we can feed it to `RemezSolver`

.

## Source code

#include "lol/math/real.h" #include "lol/math/remez.h" using lol::real; using lol::RemezSolver; real f(real const &y) { real sqrty = sqrt(y); return sin(sqrty) / sqrty; } real g(real const &y) { return re(sqrt(y)); } int main(int argc, char **argv) { RemezSolver<4, real> solver; solver.Run("1e-1000", real::R_PI_2 * real::R_PI_2, f, g, 30); return 0; }

Several subtelties here:

`sqrt(y)`

is computed only once, to save computations*f*is not defined at zero; instead of defining it, we simply reduce the range to [1e-1000; π²/4] instead of [0; π²/4]- since 1e-1000 is too small for any C++ literal number, we must use a string representation

## Compilation and execution

Build and run the above code:

make ./remez

After all the iterations the output should be as follows:

Final error: 3.325865455853290966410311141826746048362e-9 Polynomial estimate: x**0*9.999999766327004083636926544075452569800e-1 +x**1*-1.666664760824996710860650028807010146838e-1 +x**2*8.332899259989864744007300626168117151845e-3 +x**3*-1.980086431068323521164072562726168585360e-4 +x**4*2.590426526311305534315155685065532698973e-6

We can therefore write the corresponding C++ function:

double fastsin(double x) { const double a1 = 9.999999766327004083636926544075452569800e-1; const double a3 = -1.666664760824996710860650028807010146838e-1; const double a5 = 8.332899259989864744007300626168117151845e-3; const double a7 = -1.980086431068323521164072562726168585360e-4; const double a9 = 2.590426526311305534315155685065532698973e-6; return x * a1 + x*x * (a3 + x*x * (a5 + x*x * (a7 + x*x * a9))); }

Note that because of our change of variables, the polynomial coefficients are `a1`

, `a3`

, `a5`

… instead of `a0`

, `a1`

, `a2`

…

## Analysing the results

TODO (show that it was worth it)

## Conclusion

You should now be able to give hints to the minimax polynomial solver through changes of variables!

Please report any trouble you may have had with this document to sam@hocevar.net. You may then carry on to the next section: fixing parameters in the minimax polynomial.

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