20k^2-40k+10 interleaved with 20k^2-20k+10 for k>=0.

Terms

a(0) =10a(1) =10a(2) =-10a(3) =10a(4) =10a(5) =50a(6) =70a(7) =130a(8) =170a(9) =250a(10) =310a(11) =410a(12) =490a(13) =610a(14) =710a(15) =850a(16) =970a(17) =1130a(18) =1270a(19) =1450a(20) =1610a(21) =1810a(22) =1990a(23) =2210a(24) =2410a(25) =2650a(26) =2870a(27) =3130a(28) =3370a(29) =3650