a(n) is the number of 2 X 2 matrices over Z_p with determinant in {1,-1} where p = prime(n).

Terms

a(0) =6a(1) =48a(2) =240a(3) =672a(4) =2640a(5) =4368a(6) =9792a(7) =13680a(8) =24288a(9) =48720a(10) =59520a(11) =101232a(12) =137760a(13) =158928a(14) =207552a(15) =297648a(16) =410640a(17) =453840a(18) =601392a(19) =715680a(20) =777888a(21) =985920a(22) =1143408a(23) =1409760a(24) =1825152a(25) =2060400a(26) =2185248a(27) =2449872a(28) =2589840a(29) =2885568