6990447domain: NAppears in sequencesa(n) = ((n+2)/2)*Sum_{k=0..n/2}(Sum_{i=0..n-2*k} binomial(k+1,n-2*k-i)*binomial(k+i,k))/(k+1).at n=25A270715Number of circular binary sequences of length n with an even number of 0's and no three consecutive 1's.at n=26A366044