Rapid approximation of $\tanh(x)$

Question

This is kind of a signal processing/programming/mathematics crossover question. At the moment it seems more math-related to me, but if the moderators feel it belongs elsewhere please feel free to migrate it.

I'm working on a project where I have limited computational power and need to make a speedy approximation of the hyperbolic tangent function over a fairly large range of input arguments. Assuming the numbers are stored in fixed point with an 8 bit fractional part then the approximation to $\tanh(x)$ should work to the limit implied by the resolution, or for arguments $\tanh^{-1}(\pm[1 - \frac{1}{2^8}]) \approx \pm3.1$. I know that Taylor series will not converge fast enough over this range, and I haven't fully looked into it but I don't believe that a Chebyshev polynomial approximation will converge rapidly enough either. For various reasons a table lookup is also not possible.

$\tanh(x)$ is algebraically equivalent to $\mathrm{sgn}(x)(1 - \frac{2}{e^{2|x|} + 1})$. This looks promising; a series expansion of $e^x$ converges better than $\tanh(x)$. For a rapid numerical calculation on limited hardware the " + 1" in the denominator throws a wrench into the works - it means a division has to be carried out, and since many simple processors don't have "hardware divide" the division has to be done in software through repeated shifts and subtractions, which is achingly slow.

To get around this I'm considering the following approach. Instead of using $\mathrm{sgn}(x)(1 - \frac{2}{e^{2|x|} + 1})$ to exactly represent tanh(x), define the following similar type of function: $\mathrm{sgn}(x)A(1 - e^{-B|x|})$ for unknown constants A and B. Then define a loss function:

$f(x) = \int_0^{3.1}[\tanh(x) - A(1 - e^{-Bx})]^2dx$

Then use a numerical method (gradient descent?) to find the values of A and B that minimize the loss. The problem of approximating $\tanh(x)$ rapidly is then converted into approximating $e^{-x}$ rapidly plus an extra subtraction and multiplication.

$e^{-x} = \dfrac{1}{e^x} = \dfrac{1}{2^{x \log_2(e)}} = \dfrac{2^{-x}}{2^y}$, where y is the integer part and x is the fractional part. Expressing $e^{-x}$ this way lets me reduce the range of the input argument; dividing an unsigned number by $2^y$ where y is a positive integer can be accomplished by logical shifting. For arguments between 0 and -1 just eyeballing a Taylor series expansion of $2^x$ looks pretty good for just 3 terms.

Does this approach seem reasonable, from a mathematical perspective? Pitfalls I haven't considered? Thanks for any advice.

Edit: I should add that in this application (audio processing) I'm willing to trade off absolute accuracy of the calculated value in exchange for speed of processing and a function that works "reasonably" well over the entire range of input arguments - the return value for every argument in the approximation doesn't have to be accurate to the $2^{-8}$ limit implied by the resolution.

Answer 1

You are certainly aware that the hyperbolic tangent has an asymptote; since no polynomial ever had a horizontal asymptote, it stands to reason that a polynomial will always poorly approximate the qualitative behavior of the hyperbolic tangent.

One viable approach would be to consider the rational functions that come from truncations of the continued fraction for the hyperbolic tangent:

$$\tanh\,z=\cfrac{z}{1+\cfrac{z^2}{3+\cfrac{z^2}{5+\cdots}}}$$

Here for instance are plots comparing $\tanh\,z$ and the convergent

$$R(z)=\cfrac{z}{1+\cfrac{z^2}{3+\cfrac{z^2}{5+\cfrac{z^2}{7+\cfrac{z^2}{9+\cfrac{z^2}{11}}}}}}$$

The left plot shows that the rational approximant and the actual function are almost visually indistinguishable, while the right plot depicts the function $\tanh\,z-R(z)$.

One other possibility you can use in conjunction with rational function approximation is the use of argument reduction; in particular, the identity

$$\tanh\,z=\frac{2\tanh\frac{z}{2}}{1+\tanh^2\frac{z}{2}}$$

is very useful here. To sketch a possible algorithm: scale $z$ by $2^{-k}$, where $k$ is appropriately chosen such that $\dfrac{z}{2^k}$ is "small enough", evaluate the truncated continued fraction at this reduced argument, and then repeatedly apply the double argument identity $k$ times.

Answer 2

The best rational approximation to $\tanh(x)$ with numerator and denominator of degree 3 on the interval $[0, 3.1]$ (as provided by Maple's minimax function) is

(-.67436811832e-5+(.2468149110712040+(.583691066395175e-1+.3357335044280075e-1*x)*x)*x)/(.2464845986383725+(.609347197060491e-1+(.1086202599228572+.2874707922475963e-1*x)*x)*x)

This (call it $f(x)$) has maximum error .2735944241730870e-4, which is considerably less than 2^(-8).
On the interval $[-3.1, 3.1]$, use $\text{sgn}(x) f(|x|)$.

Answer 3

$\tanh(x)$ is the solution to the differential equation $y'=1-y^2$ with initial condition $y(0)=0$. There are an abundance of very fast methods for approximating solutions to autonomous differential equations like this. The most famous is Runge-Kutta 4. I feel I should emphasize - this method generally provide very good approximations in very little computing time.

Answer 4

Have you looked at CORDIC? (alas, I didn't see the no table lookup, but I'll leave it here anyway)

Answer 5

Again, in the interest of showing that there's always more than one way to skin a cat, I present a different set of rational functions that can be used instead of the continued fraction convergents I presented in my previous answer.

The idea is based on the Padé approximants of $\exp\,z$. More concretely, let the $(n,n)$ Padé approximant of $\exp\,z$ be represented by

$$\exp\,z\approx\frac{p_n(z)}{p_n(-z)}$$

where

$$p_n(z)=\sum_{j=0}^n\frac{\binom{n}{j}}{j!\binom{2n}{j}}z^j$$

From this, we find that we can approximate $\tanh\,z=\dfrac{\exp\,2z-1}{\exp\,2z+1}$ with a rational function like so:

$$\tanh\,z\approx\mathcal{T}_n(z)=\frac{p_n(z)^2-p_n(-z)^2}{p_n(z)^2+p_n(-z)^2}$$

For example,

$$\mathcal{T}_3(z)=\frac{z(10+z^2)(60+z^2)}{600+270z^2+11z^4+\frac{z^6}{24}}$$

Here are comparison plots for $\tanh\,z$ and $\mathcal{T}_3(z)$:

The left plot shows $\tanh\,z$ and $\mathcal{T}_3(z)$ together, while the right plot depicts the relative error function $1-\dfrac{\mathcal{T}_3(z)}{\tanh\,z}$. Note that the error is slightly smaller here than in the previous answer. One might still be able to improve on this approximant with a rational function with simple enough coefficients, but this is a start.

Answer 6

Surely fast and accurate methods for computing $\ln(x)$ are known. Since $\tanh^{-1}(x)=\frac12\ln\left(\frac{1+x}{1-x}\right)$, you could compute this instead, and then execute a binary search to find $\tanh(x)$. You would need to be able to accurately compute $\ln(Y)$ all the way up to $Y=\frac{1+(1-2^{-8})}{1-(1-2^{-8})}=2^9-1$. The binary search would only have $8$ repetitions.

Added later: This Wikipedia article describes a fast method for computing $\ln(Y)$ using the arithmetic-geometric mean.

Answer 7

I suggest the approach by Robert Israel. I have recently done something similar to approximate this function, so here are my two cents. I was actually looking for an approximation of the logistic function, which is just a scaling and translation of this, but the hyperbolic tangent is easier to work with because it is symmetric around the 0. My model calculates tanh(x/2).

In my case I found a higher degree approximation that kept the maximum absolute error below 2e-7. I would like to know if Maple can deal with higher order models, because there is the challenge of avoiding to put poles over the real line. In fact, this model suggested by Robert has a pole approximately at -3.8, that may not be a problem in this application, but this is not great to have...

In my models the numerator is an odd polynomial, and the denominator is even, so all poles and zeros are imaginary pairs, with an extra zero at the origin. Here is the big model I found, for an error below 2e-7:

y =  94.9339451088 * x * ((( x**2 + 1.06315869e+03 ) * x**2 + 1.88748783e+05 ) * x**2 + 5.86237309e+06 ) / ((( ( x**2 + 4.03183926e+03 ) * x**2 + 1.64253046e+06 ) * x**2 + 1.28592857e+08 ) * x**2 + 1.11307745e+09 )

A smaller order model for ~2**-8 error can also certainly be found with the same approach. I used my code to make a new attempt to find this simpler model, and this is what I got, check it out:

y = 25.2075466924 * x * ( x**2 + 6.96321678**2 ) / (( x**2 + 3.17572786**2 ) * ( x**2 + 15.57611343**2 ))

This approximates tanh(x/2) with an error slightly above 2.2e-4 for |x| < 7.6. The derivative of the approximated function at 7.6 also equals 0, so it's safe to truncate the function there (make it sgn(x)), while the resulting model will become monotonically increasing, a nice property to have.

I could not get a good model for an even smaller order, but I don't believe it can get much simpler than that.

PS: The continued fraction x / (1+x^2/(3+x^2/(5+...))) also produces a very similar rational function, but the error is just very flat around 0, different from the Chebyshev-style fit I did. I would like to know if we could use that continued fraction to come up with a good initial estimate for the non-linear regression...