-21*zeta_K(-1), where K runs through the simplest cubic fields.

Terms

a(0) =1a(1) =7a(2) =21a(3) =147a(4) =1393a(5) =2569a(6) =7525a(7) =13748a(8) =30171a(9) =43491a(10) =100639a(11) =123456a(12) =224175a(13) =296991a(14) =495367a(15) =658684a(16) =1013008a(17) =1438440a(18) =1954357a(19) =2310462a(20) =3813903a(21) =4092029a(22) =5916929a(23) =6974592a(24) =10276475a(25) =10989267a(26) =14919105a(27) =21957309a(28) =26812232a(29) =36865003