a(n) = Sum_{k=0..n} sigma(k^2 + 1), where sigma(k) is the sum of divisors of k (A000203).

Terms

a(0) =1a(1) =4a(2) =10a(3) =28a(4) =46a(5) =88a(6) =126a(7) =219a(8) =303a(9) =429a(10) =531a(11) =717a(12) =897a(13) =1221a(14) =1419a(15) =1761a(16) =2019a(17) =2559a(18) =2993a(19) =3539a(20) =3941a(21) =4697a(22) =5285a(23) =6257a(24) =6835a(25) =7777a(26) =8455a(27) =9787a(28) =10735a(29) =12001