Negative values along the main diagonal of the array defined by A020806 and its differences.

Terms

a(0) =-1a(1) =2a(2) =9a(3) =13a(4) =31a(5) =57a(6) =119a(7) =233a(8) =471a(9) =937a(10) =1879a(11) =3753a(12) =7511a(13) =15017a(14) =30039a(15) =60073a(16) =120151a(17) =240297a(18) =480599a(19) =961193a(20) =1922391a(21) =3844777a(22) =7689559a(23) =15379113a(24) =30758231a(25) =61516457a(26) =123032919a(27) =246065833a(28) =492131671a(29) =984263337