a(n) is the total sum of semiperimeters over all (>=,>=)-polyominoes of length n.

Terms

a(0) =0a(1) =2a(2) =7a(3) =21a(4) =62a(5) =180a(6) =522a(7) =1512a(8) =4384a(9) =12726a(10) =36995a(11) =107701a(12) =313986a(13) =916604a(14) =2679159a(15) =7840125a(16) =22967784a(17) =67352334a(18) =197693325a(19) =580775223a(20) =1707553410a(21) =5024194308a(22) =14793209508a(23) =43585511382a(24) =128495325672a(25) =379036691250