Short Version : You need an expert in Manure, not mathematics.
In such a post-neolithic world (e.g early bronze age), your mathematician starts of :
- Probably older than everyone else - average life expectancy would be about 25 at that time and even a new graduate in mathematics would be about 22.
- A babbling idiot who could not communicate in any locally recognizable language
- Potentially lethal to the locals as his body carries diseases internally their immune systems would be very vulnerable to.
- Incapable of feeding, clothing or hunting in the way the locals do (if at all).
- Socially inept to the point that they he cold easily cross a local social or religious taboo and find himself dead (or worse). Such a culture would be essentially unrecognizable to a modern human and they would be lucky to navigate through it socially without disaster after disaster.
- Repelled by local drinking water. Ever actually look at a glass of water from a river ? Don't drink it unless you're feeling lucky - no clean tap water in those days.
So your mathematician would start out, not as a wise man, but as a useless, incompetent misanthrope and fool.
And as survival was a day-to-day struggle and charity a luxury at that time, I'd say the odds of your mathematician surviving long enough to learn basic survival skills and communicate with the locals is very small.
But lets say they do ...
What can they teach the locals to gain their respect ?
Not bloody mathematics that's for sure !
Useful skills to neolithic cultures :
- Manure Science - what can you do with manure ?
- Maybe basic hygiene practices - can you mathematician make basic soap ?
- Calender skills - can your mathematician remember enough basic astronomy to help the locals predict the seasons accurately (useful for agriculture) ?
- Maybe mapping and surveying skills.
- The use of a plumb bob.
Ancient cultures has little or no practical use for mathematics (note that the concept of a number zero did not appear until about 300 BC and negative numbers were not developed until about 200 BC).
Again remember that survival is the only focus of day to day activity and long term planning means crop gathering, planting and rotation and the storage of grain and other food.
Lets say your mathematician strikes it lucky and comes to the attention of the equivalent of scholars of the time and gets to show some skills (area of a circle, geometric proof processes). He might get to teach some basic skills in symbolic algebra and maybe elementary calculus. But again remember that applying these skills is not the same as knowing them. It's quite possible he could teach e.g. twenty people these (for the time) very abstract and esoteric studies, but would they have any impact long term ?
It's not like he could write a book.
Now we're talking.
Your chap comes to the attention of high and mighty and, more to the point, the early equivalent of bureaucrats, and knowledge that you could print in a structured way (typeface) would be really useful them.
If your chap forgot the maths and simply got around to explaining how to print and keep records on paper (how do you make basic paper) or even on cloth, your chaps would bring the printed word (and the ability to educate and organize and record) to the world very early on.
Woodblock printing did not appear until something like 200 AD, so if your mathematician got that going in the 4000 BC range, it would certainly have helped speed up development of many things (like the invention of taxation and revenue collection - yay !)
Now assuming your chap helps develop printing he's also now in a position to plagiarize the work of every mathematician he can remember and pass it on to what would become a very different (but probably equally bloody) history !
Another Mathematician becomes a Bureaucrat
Your chap will now follow (eh, precede) Issac Newton as a government employee par excellence by helping them become better (the first !) paper pushers and probably also help the local head honcho realize the benefits of a sound economy based on tax collection (and the need for regrettably violent punishments for avoidance and tax exemption for senior government figures and researchers in science and engineering).
And this is a good thing from your chaps point of view as he'd starve to death otherwise !
A Ripe Old Age
He could live as old as 35, maybe even 40 if he plays his cards right.
But only if his health care reforms (like soap and cleaner-than-mud drinking water) come into vogue.