typed in hollow ball

https://www.amazon.com/Bright-Starts-Oball-Easy-Grasp-Classic-Ball-BPA-Free-Age-Newborn/dp/B00ZRD99C0/ref=asc_df_B00ZRD99C0/?tag=&linkCode=df0&hvadid=343257911461&hvpos=&hvnetw=g&hvrand=17929977158879662036&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9003367&hvtargid=pla-377551567593&ref=&adgrpid=67962871134&th=1

I don't have a definite answer for you. Googling this question really only brings up theoretical math equations. If you are looking in real life, one place to start is looking at these toys:

Hoberman Sphere .

Nerf Ball

Here is a baby toy called an Easy-Grasp ball.

And here is a furniture decoration, which is not soft and likely has no "give".

None of these is exactly what you are looking for, but I hope they help you or a later commenter to track it down further.

This was probably the best toy that we were gifted: the Oball