First differences of A037257.

Terms

a(0) =2a(1) =6a(2) =11a(3) =18a(4) =26a(5) =36a(6) =48a(7) =61a(8) =75a(9) =90a(10) =106a(11) =123a(12) =142a(13) =163a(14) =185a(15) =208a(16) =232a(17) =257a(18) =285a(19) =314a(20) =344a(21) =375a(22) =407a(23) =440a(24) =474a(25) =509a(26) =546a(27) =585a(28) =625a(29) =666