15. One of the biggest mathematical breakthroughs in 2019 was progress on an 82 -year old problem by the renowned mathematician and Fields medalist Terence Tao.
Consider the function
$$
f(n)=\left\{\begin{array}{cl}
\frac{n}{2} & \text { if } n \text { is even } \\
3 n+1 & \text { if } n \text { is odd }
\end{array}\right.
$$

Starting with any positive integer $n$, it was conjectured that recursive applications of the above function always lead to 1 .

While a general proof of this result still eludes the mathematical community, Tao was able to show that if there are counterexamples to this conjecture, their frequency approaches 0 as $n$ increases. What is the surname of the German mathematician who proposed this conjecture in $1937 ?$
Answer: Collatz