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

Terms

a(0) =7a(1) =35a(2) =112a(3) =315a(4) =847a(5) =2240a(6) =5887a(7) =15435a(8) =40432a(9) =105875a(10) =277207a(11) =725760a(12) =1900087a(13) =4974515a(14) =13023472a(15) =34095915a(16) =89264287a(17) =233696960a(18) =611826607a(19) =1601782875a(20) =4193522032a(21) =10978783235a(22) =28742827687a(23) =75249699840a(24) =197006271847a(25) =515769115715