4103712domain: NAppears in sequencesNumber of length-n strings w over a 4-letter alphabet with the property that if x is a subword of w and |x| >= 2, then x reversed is not a subword of w.at n=33A330011a(n) = n!*Sum_{m=1..floor((n+1)/2)} 1/(m*binomial(n-m,m-1)).at n=9A358652