Bug with cosine of an interval

[haavarn]

Hi,
There seems to be a bug with the cosine function. Unfortunately, I have not been able to spot the error within the source code myself...

Here is the infimum and supremum of a cosine with an interval +/- 10 degs around the nominal:

You can generate the plots using:

angs = -2*pi:0.1:2*pi;
sup = zeros(size(angs));
inf = zeros(size(angs));
 
diff = 10 / 180*pi;
 
for i = 1:length(angs)
    sup(i) = cos(interval( angs(i)-diff, angs(i)+ diff)).supremum;
    inf(i) = cos(interval( angs(i)-diff, angs(i)+ diff)).infimum;
end

plot(angs,inf)
plot(angs,sup)

The sine is fine, so it is possible to use cos(interval) = sin(pi/2 – interval)

Cheers,
Håvard