57732domain: NAppears in sequencesNumber of active (ON, black) cells at stage 2^n-1 of the two-dimensional cellular automaton defined by "Rule 121", based on the 5-celled von Neumann neighborhood.at n=7A270207G.f.: Sum_{k>=0} x^(k*(k+1)) * Product_{j=1..k} ((1 + x^j)/(1 - x^j))^2.at n=31A376852