Third differences of A001521.

Terms

a(0) =0a(1) =1a(2) =0a(3) =0a(4) =1a(5) =0a(6) =1a(7) =2a(8) =2a(9) =2a(10) =4a(11) =6a(12) =8a(13) =10a(14) =16a(15) =22a(16) =31a(17) =44a(18) =62a(19) =88a(20) =124a(21) =176a(22) =248a(23) =352a(24) =497a(25) =702a(26) =994a(27) =1406a(28) =1987a(29) =2812