a(2) = 1;
+--+
| |
+--+
a(3) = 1;
+--+--+
| |
+ +
| |
+--+--+
a(4) = 11;
+--+--+--+ +--+--+--+ +--+--+--+
| | | | | |
+--*--* + +--* *--+ +--* +
| | | | | |
+--*--* + +--* *--+ +--* +
| | | | | |
+--+--+--+ +--+--+--+ +--+--+--+
+--+--+--+ +--+--+--+ +--+--+--+
| | | | | |
+ *--*--+ + *--* + + *--+
| | | | | | | |
+ *--*--+ + * * + + *--+
| | | | | | | |
+--+--+--+ +--+ +--+ +--+--+--+
+--+--+--+ +--+--+--+ +--+ +--+
| | | | | | | |
+ + + + + *--* +
| | | | | |
+ *--* + + + + *--* +
| | | | | | | | | |
+--+ +--+ +--+--+--+ +--+ +--+
+--+ +--+ +--+ +--+
| | | | | | | |
+ *--* + + * * +
| | | | | |
+ + + *--* +
| | | |
+--+--+--+ +--+--+--+
|