a(n) = (7^n - 6^n - 5^n - 4^n + 4*3^n)/2.

Terms

a(0) =1a(1) =2a(2) =4a(3) =23a(4) =274a(5) =2927a(6) =27094a(7) =228923a(8) =1827634a(9) =14069687a(10) =105715534a(11) =781107923a(12) =5702856394a(13) =41273440847a(14) =296753044774