(11*9^n-1)/2.

Terms

a(0) =5a(1) =49a(2) =445a(3) =4009a(4) =36085a(5) =324769a(6) =2922925a(7) =26306329a(8) =236756965a(9) =2130812689a(10) =19177314205a(11) =172595827849