a(n) = (3*n)^7.

Terms

a(0) =0a(1) =2187a(2) =279936a(3) =4782969a(4) =35831808a(5) =170859375a(6) =612220032a(7) =1801088541a(8) =4586471424a(9) =10460353203a(10) =21870000000a(11) =42618442977a(12) =78364164096a(13) =137231006679a(14) =230539333248a(15) =373669453125a(16) =587068342272a(17) =897410677851