C2

Part of 2021-IMOC qualification

Problems(1)

Large integer without repeating abab

Source: 2021 IMOC qualification problems, C2

Find the largest positive integer

n

such that no two adjacent digits are the same, and for any two distinct digits

0 \leq a,b \leq 9

, you can't get the string

abab

just by removing digits from

n

IMOCcombinatorics