Real zeros of sine Taylor polynomials

The more terms of Taylor series {\displaystyle \sin x = x-\frac{x^3}{3!}+ \frac{x^5}{5!}- \cdots } we use, the more resemblance we see between the Taylor polynomial and the sine function itself. The first-degree polynomial matches one zero of the sine, and gets the slope right. The third-degree polynomial has three zeros in about the right places.

Third degree Taylor polynomial
Third degree, three zeros

The fifth-degree polynomial will of course have … wait a moment.

Fifth degree Taylor polynomial
Fifth degree, only one zero

Since all four critical points are in the window, there are no real zeros outside of our view. Adding the fifth-degree term not only fails to increase the number of zeros to five, it even drops it back to the level of {T_1(x)=x}. How odd.

Since the sine Taylor series converges uniformly on bounded intervals, for every { A } there exists { n } such that {\max_{[-A,A]} |\sin x-T_n(x)|<1 }. Then { T_n } will have the same sign as { \sin x } at the maxima and minima of the latter. Consequently, it will have about { 2A/\pi } zeros on the interval {[-A,A] }. Indeed, the intermediate value theorem guarantees that many; and the fact that {T_n'(x) \approx \cos x } on { [-A,A]} will not allow for extraneous zeros within this interval.

Using the Taylor remainder estimate and Stirling's approximation, we find {A\approx (n!)^{1/n} \approx n/e }. Therefore, { T_n } will have about { 2n/(\pi e) } real zeros at about the right places. What happens when {|x| } is too large for Taylor remainder estimate to be effective, we can't tell.

Let's just count the zeros, then. Sage online makes it very easy:

sineroots = [[2*n-1,len(sin(x).taylor(x,0,2*n-1).roots(ring=RR))] for n in range(1,51)]
scatter_plot(sineroots) 
Roots of sine Taylor polynomials
Roots of sine Taylor polynomials

The up-and-down pattern in the number of zeros makes for a neat scatter plot. How close is this data to the predicted number { 2n/(\pi e) }? Pretty close.

scatter_plot(sineroots,facecolor='#eeee66') + plot(2*n/(pi*e),(n,1,100))
Compared to 2n/pi*e
Compared to 2n/pi*e

The slope of the blue line is { 2/(\pi e) \approx 0.2342 }; the (ir)rationality of this number is unknown. Thus, just under a quarter of the zeros of { T_n } are expected to be real when { n } is large.

The actual number of real zeros tends to exceed the prediction (by only a few) because some Taylor polynomials have real zeros in the region where they no longer follow the function. For example, { T_{11} } does this:

Spurious zero around x=7
Spurious zero around x=7

Richard S. Varga and Amos J. Carpenter wrote a series of papers titled Zeros of the partial sums of { \cos z } and {\sin z } in which they classify real zeros into Hurwitz (which follow the corresponding trigonometric function) and spurious. They give the precise count of the Hurwitz zeros: {1+2\lfloor n/(\pi e)\rfloor } for the sine and {2\lfloor n/(\pi e)+1/2\rfloor } for the cosine. The total number of real roots does not appear to admit such an explicit formula. It is the sequence A012264 in the OEIS.

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