a(n) is the number of isosceles trapezoids in a triangular grid of side length n.

Terms

a(0) =0a(1) =3a(2) =18a(3) =57a(4) =138a(5) =282a(6) =516a(7) =870a(8) =1380a(9) =2085a(10) =3030a(11) =4263a(12) =5838a(13) =7812a(14) =10248a(15) =13212a(16) =16776a(17) =21015a(18) =26010a(19) =31845a(20) =38610a(21) =46398a(22) =55308a(23) =65442a(24) =76908a(25) =89817a(26) =104286a(27) =120435a(28) =138390a(29) =158280