Math problems of last semester in senior one.

Solution: ∫s = {0, 1, 2, 3, 4, 5},

The four elements of a collection without orphans must be:

* * * There are {0, 1, 2,3}, {0, 1, 3,4}, {0, 1, 4,5}, {1, 2,3,4}, {

Then the number of subsets A of four elements without "isolated elements" in S is 6.

If there is 0, there must be 1, and if there is 5, there must be 4.

If there is no 0,5, then 1 2 3 4 meets the requirements.

If there is 0, 1, the remaining two elements can be: (2,3) (3,4) (4,5).

If there are 4,5, the remaining two elements can be described as: (0, 1) (1, 2) (2,3).

To sum up, subtract a group of repeated 0 145, ***6 groups.