a(n) = binomial(n,27).

Terms

a(0) =1a(1) =28a(2) =406a(3) =4060a(4) =31465a(5) =201376a(6) =1107568a(7) =5379616a(8) =23535820a(9) =94143280a(10) =348330136a(11) =1203322288a(12) =3910797436a(13) =12033222880a(14) =35240152720a(15) =98672427616a(16) =265182149218a(17) =686353797976