See: List of castles, Castles in England, Castles in Scotland, Castles in Northern Ireland
Anglesey
Clwyd
Dyfed
Glamorgan
Gwent
Gwynedd
Powys
South Glamorgan
West Glamorgan
... assumed to be real (but see below for generalizations) with a being non-zero. These solutions are also called the roots of the equation. The formu ...