A034186 Number of symmetric n X 4 crossword puzzle grids.
1, 3, 21, 47, 227, 499, 2321, 5087, 23429, 51263, 235881, 515691, 2372511, 5185609, 23856215, 52139887, 239863701, 524238555, 2411688233, 5270900439, 24248031407, 52995614649
Offset: 1