Number of symmetric n X 4 crossword puzzle grids.

Terms

a(0) =1a(1) =3a(2) =21a(3) =47a(4) =227a(5) =499a(6) =2321a(7) =5087a(8) =23429a(9) =51263a(10) =235881a(11) =515691a(12) =2372511a(13) =5185609a(14) =23856215a(15) =52139887a(16) =239863701a(17) =524238555a(18) =2411688233a(19) =5270900439a(20) =24248031407a(21) =52995614649