25488736domain: NAppears in sequencesa(n) = Sum_{k=0..floor(n/7)} binomial(n-4*k,3*k).at n=42A370722Number of compositions of 7*n into parts 3 and 7.at n=18A373909a(n) = Sum_{k=0..floor(2*n/7)} binomial(2*n-4*k,3*k).at n=21A392403