Largest prime factors of two consecutive natural numbers

Question

Let $u(n)$ be the numbers of positive integers $k \le n$ such that the largest prime factor of $k+1$ is greater than the largest prime factor of $k$.

Similarly, let $l(n)$ be the numbers of positive integers $k \le n$ such that the largest prime factor of $k+1$ is smaller than the largest prime factor of $k$.

As we go higher up the number line, the density of primes decreases in accordance with the Prime Number Theorem. Since more natural numbers to be composed of small prime factors than with large prime factors I expected $u(n)$ to be much greater than $l(n)$. However, computation showed that their difference is much lesser than I expected. In fact,we have: $$ u(10000) = 5008, l(10000) = 4991 $$ $$ u(100000) = 50079, l(100000) = 49920 $$

In other words, even though density of primes thins out, natural numbers are such that the largest prime factors of two consecutive natural numbers are almost equally likely to greater of less than one another.

Question: What is the expected gap between $u(n)$ and $l(n)$ as per the Prime Number theorem?

Answer 1

Although you expect the largest prime factor of a $n$ to generally trend bigger as $n$ increases, first of all:

• that growth is slow and highly erratic
• perhaps more important for your puzzle, one of $k$ and $k+1$ is even, the other is odd. You'd kind of expect the odd number to have larger prime factors. Hence your $u(n)$ and $l(n)$ will be similar.

You could try defining $u(n)$ and $l(n)$ in terms of $k$ and $k+2$, to see if you get any interesting difference between them. I suspect not, but you never know.

In any case, testing up to small $n$ such as $n=10000$ tells you very little about long-term trends: see this article for example.