Pairwise sums of general ballot numbers (A002026).

Terms

a(0) =1a(1) =3a(2) =7a(3) =17a(4) =42a(5) =106a(6) =272a(7) =708a(8) =1865a(9) =4963a(10) =13323a(11) =36037a(12) =98123a(13) =268737a(14) =739833a(15) =2046207a(16) =5682915a(17) =15842505a(18) =44315637a(19) =124348275a(20) =349911204a(21) =987212856a(22) =2791964574a(23) =7913642086a(24) =22477090679a(25) =63964370301a(26) =182353459733a(27) =520735012027