 ## Long substring with many repetitions Published on Sunday, 1st December 2019, 10:00 am; Solved by 140

### Problem 691

Given a character string $s$, we define $L(k,s)$ to be the length of the longest substring of $s$ which appears at least $k$ times in $s$, or $0$ if such a substring does not exist. For example, $L(3,\text{“bbabcabcabcacba”})=4$ because of the three occurrences of the substring $\text{“abca”}$, and $L(2,\text{“bbabcabcabcacba”})=7$ because of the repeated substring $\text{“abcabca”}$. Note that the occurrences can overlap.

Let $a_n$, $b_n$ and $c_n$ be the $0/1$ sequences defined by:

• $a_0 = 0$
• $a_{2n} = a_{n}$
• $a_{2n+1} = 1-a_{n}$
• $b_n = \lfloor\frac{n+1}{\varphi}\rfloor - \lfloor\frac{n}{\varphi}\rfloor$ (where $\varphi$ is the golden ratio)
• $c_n = a_n + b_n - 2a_nb_n$

and $S_n$ the character string $c_0\ldots c_{n-1}$. You are given that $L(2,S_{10})=5$, $L(3,S_{10})=2$, $L(2,S_{100})=14$, $L(4,S_{100})=6$, $L(2,S_{1000})=86$, $L(3,S_{1000}) = 45$, $L(5,S_{1000}) = 31$, and that the sum of non-zero $L(k,S_{1000})$ for $k\ge 1$ is $2460$.

Find the sum of non-zero $L(k,S_{5000000})$ for $k\ge 1$.