a(n) = 3*7^n+1.

Terms

a(0) =4a(1) =22a(2) =148a(3) =1030a(4) =7204a(5) =50422a(6) =352948a(7) =2470630a(8) =17294404a(9) =121060822a(10) =847425748a(11) =5931980230a(12) =41523861604a(13) =290667031222