How many subsets are there of size k from the set {1,2,...n} such that if a subset contains 2 it doesn't contain 1?
My answer:
The total number of possible subsets of size k is just $|\Omega| ={n \choose k}$. Let $A_k$ be subset of size $k$ and divide by cases:
- i) subset contains both 1 and 2
- ii) subset containers neither 1 nor 2
- iii) subset contains 1 but not 2, or 2 but not 1 (same amount for both cases)
In other words, $$ |\Omega| = |1,2\in A_k| + |1,2\notin A_k| +2 |1\in A_k,2\notin A_k| $$.
For case i), we know that $1,2\in A_k$, so we count how many subsets of size $k-2$ there are from set $\{3,4,...n\}$, which is ${n-2} \choose {k-2}$. For case ii), we have to choose $k$ elements from $\{3,4,....n\}$, which gives $n-2 \choose k$ options. Thus, my answer would be
$$\dfrac{1}{2} ({n \choose k} - {n-2 \choose k} - {n-2\choose k-2})$$
The actual solution isn't given, so I don't know if my answer is correct or not. Any help?