How many minimum trips are needed?

Question

There is an Island of puzzles called puzzles-world where numbers 1 - 9 want to cross a river.
There is a single boat that can take numbers from one side to the other.
However, maximum 3 numbers can go at a time and of course,
the boat cannot sail on its own so one number must come back after reaching to another side.
Also, the sum of numbers crossing at a time must be a square number.
You need to plan trips such that minimum trips are needed.

How many minimum trips are needed ?

Answer 1

4 trip from island to other side is enough.
Let me give representation :
1,2,3,4,5,6,7,8,9 there at island initially.

Transit=》sum
1,2,6 sums to 9,a square transit,
1 a square comes back,
1 3 5 summing 9 transit,
1 3 summing 4 comes back,
1 7 8 summing 16 transit,
1 comes back,
3 4 9 summing 16 transit
1,2,3,4,5,6,7,8,9 there at other end now.