180732domain: NAppears in sequencesNumber of (n+1)X(n+1) -10..10 symmetric matrices with every 2X2 subblock having sum zero and three distinct values.at n=11A211814Numbers k such that 7*R_k - 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.at n=18A256830