# Combination under constrained situation with a condition

Step 1: I want to take a number `n`

as input from user

Step 2: We form the set `S`

consisting of elements from `0`

to `n*(2^{n-1})`

Step 3: Now I pick each possible two-element subsets of `S`

and store it in `P`

.

Step 4: Now I need to pick `n*(2^{n-1})`

two-element subsets from `P`

such that each number that occurs in that set occurs exactly `n`

times
neither less nor more and put them all in a list.

Example

```
n = 2
n*(2^{n-1}) = 2*(2^{2-1}) = 4
S = {0,1,2,3,4}
p = {(0,1),(0,2),(0,3),(0,4),(1,2),(1,3),(1,4),(2,3),(2,4),(3,4)}
```

Step 4 One element which satisfies condition of step 4; HERE `n = 2`

.

`{(0,1),(1,2),(2,3),(0,3)}`

which has `2*(2^{n-1}) = 2*(2^{2-1}) = 4`

elements.

Now see in the above set

`0`

occurred`n=2`

times only in`(0,1)`

and`(0,3)`

only`1`

occurred`n=2`

times only in`(0,1)`

and`(1,2)`

only`2`

occurred`n=2`

times only in`(1,2)`

and`(2,3)`

only`3`

occurred`n=2`

times only in`(2,3)`

and`(0,3)`

only

Similarly we may get for `{(0,1),(1,4),(4,2),(2,0)}`

we can easily verify like above.

Now based on `n`

the number of elements size etc will vary.

Kindly help if possible any one.