12582905domain: NAppears in sequencesNumber of active (ON, black) cells at stage 2^n-1 of the two-dimensional cellular automaton defined by "Rule 251", based on the 5-celled von Neumann neighborhood.at n=11A271017a(2*n) = 9*2^n - 7, a(2*n+1) = 3*2^(n+2) - 7.at n=41A354789