[問題] 特南羅馬尼亞題

[Dawsen (阿布拉克薩斯)]

A word consists of n letters from the alphabet
{a,b,c,d}. Aword is called convoluted if it has two
consecutive identical blocks of letters. For
example,
caab and cababdc are convoluted, but abcab is not.
Prove that the number of non-convoluted words with
n letters is greater than 2^n

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