110579domain: NAppears in sequencesNumber of interior vertices formed by drawing the lines connecting any two of the 2*(n+2) perimeter points of a 3 X (n+1) square grid.at n=27A331767a(n) = [x^n] 2 / (1 - 7*x + sqrt(1 - 2*x - 3*x^2)).at n=8A344506