3 cell arrow has restrictions on the head - must be at least 1+2=2. Similarly along the arrow, you can't have a 9 on a 3 cell arrow.
There's the concept of unique groups. Partition the cells on an arrow [or others] such that each partition contains the cells that must be unique. If they are unique, we know the minimum value as the triangular number for n the size of the partition. Add up all such n and you get the minimum value of the arrow head.
The unique groups is also needed to restrict frozen thermometers
2 cell arrows mean head = tail.
3 cell arrow has restrictions on the head - must be at least 1+2=2. Similarly along the arrow, you can't have a 9 on a 3 cell arrow.
There's the concept of unique groups. Partition the cells on an arrow [or others] such that each partition contains the cells that must be unique. If they are unique, we know the minimum value as the triangular number for n the size of the partition. Add up all such n and you get the minimum value of the arrow head.
The unique groups is also needed to restrict frozen thermometers