a(n) = binomial(n+3, 3)*7^n.

Terms

a(0) =1a(1) =28a(2) =490a(3) =6860a(4) =84035a(5) =941192a(6) =9882516a(7) =98825160a(8) =951192165a(9) =8877793540a(10) =80787921214a(11) =719746934452