a(n) = sum of the numbers between the two n's in A026242.

Terms

a(0) =0a(1) =3a(2) =6a(3) =14a(4) =25a(5) =33a(6) =49a(7) =60a(8) =81a(9) =105a(10) =121a(11) =150a(12) =182a(13) =203a(14) =240a(15) =264a(16) =306a(17) =351a(18) =380a(19) =430a(20) =462a(21) =517a(22) =575a(23) =612a(24) =675a(25) =741a(26) =783a(27) =854a(28) =899a(29) =975