a(2)=17:
.___. .___. .___. .___. .___. .___. .___. .___. .___.
| | | |___| |___| | | | |___| |___| | | | | ._| |_. |
| | | | | | |___| |_|_| | | | |___| |_|_| |_| | | |_|
|_|_| | | | |___| |___| |_|_| | | | | | | |___| |___|
|___| |_|_| |___| |___| |___| |_|_| |_|_| |___| |___|
.
.___. .___. .___. .___. .___. .___. .___. .___.
|___| |___| | | | | | | |_. | | ._| |_. | | ._|
| ._| |_. | | |_| |_| | | |_| |_| | | |_| |_| |
|_| | | |_| |_| | | |_| | | | | | | |_| | | |_|
|___||___ |___ |___ |___ |_|_|_|_|_|_|___ |___ |___ |。
|