Einstein also believed that the formula for compounding interest was the greatest mathematical discovery of all time. 

Let's try a mathematical simulation to see if @Scott Trench is wrong.Lets say you have $20,000 and buy a $100,000 rental property using banks money.

I'm an engineer not a mathematician so this may not be the most technical of descriptions, but should convey the general idea....in layman's term: the mortgage payment is mathematically determined based on your interest payment being logarithmic (i.e. it reduces each month) and your principal payment being exponential (it increases each month) such that the total payment stays the same. 

Mathematically I doubt there is anything that comes remotely close to 2nd place as "unrealized". 

If you're looking at large exchange-traded companies the market does a pretty good job of pricing assets and data can yield volatility, variance, etc. figures that would aid in establishing mathematical defintions of risk for a given asset.

If you want to learn about technology I️ would Learn hard skills like coding or Mathematics.

It is a mathematical certainty that socialism will double the price of Seattle real estate.

The issue is not whether it's mathematically correct, but rather whether it is appropriate.

But a scientific theory cannot ever be proven true for all real world conditions.Only mathematical theorems can be proven (in fact, the formal process of doing this is called a "proof").It can be shown mathematically that under specific conditions that if the increase in the price of any limited good outpaces inflation long-term, it will become unafforable.  

Compare this to heuristic, a similar concept used in mathematical discourse, psychology and computer science, particularly in algorithm design.There are so many MORE expenses and benefits to a rental property that are not even covered in the 50% "rule of thumb."