User blog:Primussupremus/Harvey Friedmans word puzzle.

So I was watching some of the David Metzler videos people were telling me about and I came across a puzzle in the Video called Harvey Friedmans word puzzle. The rules are simple you have to make the longest string of letters/word possible without any substrings having any matches to another substring. For example abbaaabb is illegal as it has two matches but abbbaaaaaaa is legal it has been found that the longest string you can make with one letter/symbol is aaa or bbb etc because if you cross out or remove symbols you won't get pairs aaa=aa,a see no matches. My question is what is the longest string possible so that no matches occur using all 26 letters of the English alphabet?