a(1) = 0; for n > 1 a(n) = sum of divisors of n^2-1; or sigma(A005563(n-1)).

Terms

a(0) =0a(1) =4a(2) =15a(3) =24a(4) =60a(5) =48a(6) =124a(7) =104a(8) =186a(9) =156a(10) =360a(11) =168a(12) =480a(13) =336a(14) =504a(15) =432a(16) =819a(17) =360a(18) =1170a(19) =640a(20) =1080a(21) =768a(22) =1488a(23) =744a(24) =1736a(25) =1240a(26) =1680a(27) =1200a(28) =2880a(29) =960