508373165666domain: NAppears in sequencesNumber of black-square subarrays of (n+2) X (2+2) 0..3 arrays x(i,j) with each element diagonally or antidiagonally next to at least one element with value (x(i,j)+1) mod 4, no adjacent elements equal, and upper left element zero.at n=24A230929a(n) = round(3^n/5).at n=26A383324