Shortest Palindromic Superstring

A palindrome is a string that is identical to its reverse. For example, "aba", "abba", "a" and "aa" are palindromes, while "ab", "abb" and "xyz" are not. A string $X$ is called a substring of string $Y$ if it is possible to obtain $X$ by erasing some (possibly zero) characters from the beginning of $Y$ and some (possibly zero) from the end of $Y$ without changing the order of the remaining charac