a(n) = 4*a(n-1) - 4*a(n-2) + a(n-3), n >= 3; a(0)=1, a(1)=6, a(2)=20.

Terms

a(0) =1a(1) =6a(2) =20a(3) =57a(4) =154a(5) =408a(6) =1073a(7) =2814a(8) =7372a(9) =19305a(10) =50546a(11) =132336a(12) =346465a(13) =907062a(14) =2374724a(15) =6217113a(16) =16276618a(17) =42612744a(18) =111561617a(19) =292072110a(20) =764654716a(21) =2001892041a(22) =5241021410a(23) =13721172192a(24) =35922495169a(25) =94046313318a(26) =246216444788