[Speex-dev] math_approx.c

[Oleg Homenko]

Hi,
I started to optimize math_approx fixed point functions to ARM CPU. And 
found out that approximations are not Taylor series, for example:
spx_exp2(x) = D0 + D1*x + D2*x^2 +D3*x^3,
where D0 = 1, D1 = log(2), D2 = (log(2))^2 / 2, D3 = (log(2))^3 / 6 for 
Taylor serie,
but Speex source code uses different coefficients:
D0 = 1, D1 = log(2), D2 = 3 - 4*log(2), D3 = 3*log(2) - 2

Can anyone explain the difference? Was it done to equalize error along 
range? How were obtained D2 and D3?
Thanks,
Oleg Homenko