671856domain: NAppears in sequencesa(n) = Sum_{k=0..floor(n/7)} C(n-5*k,2*k).at n=42A098574Number of compositions of 7*n into parts 2 and 7.at n=12A373908a(n) = Sum_{k=0..floor(2*n/7)} binomial(2*n-5*k,2*k).at n=21A392398