Number of Schur rings over Z_{3^n}.

Terms

a(0) =1a(1) =2a(2) =7a(3) =25a(4) =92a(5) =345a(6) =1311a(7) =5030a(8) =19439a(9) =75545a(10) =294888a(11) =1155205a(12) =4538745a(13) =17876250a(14) =70553179a(15) =278949705a(16) =1104585634a(17) =4379770585a(18) =17386456213a(19) =69090680674a(20) =274806384941