G.f. = { 1+sum(4*n*q^n, n=1..infinity)} / { theta series for square lattice }.

Terms

a(0) =1a(1) =0a(2) =4a(3) =-4a(4) =12a(5) =-20a(6) =40a(7) =-68a(8) =124a(9) =-208a(10) =352a(11) =-580a(12) =944a(13) =-1508a(14) =2392a(15) =-3732a(16) =5772a(17) =-8836a(18) =13396a(19) =-20132a(20) =30024a(21) =-44412a(22) =65240a(23) =-95188a(24) =137984a(25) =-198808a(26) =284816a(27) =-405764a(28) =575072a(29) =-810980