HMMT Combinatorics 2019/1: Palindromes
Source:
February 17, 2019
HMMT
Problem Statement
How many distinct permutations of the letters in the word REDDER are there that do not contain a palindromic substring of length at least two? (A substring is a continuous block of letters that is part of the string. A string is palindromic if it is the same when read backwards.)