thanks for response, gutu is right

Case1:
monday - 10k invested on 'A'
thursday - sold 'A' completely
thursday(same day )- cannot buy 'A' but can buy 10k worth of 'B' but cannot sell 'B' (else it )
Not correct. You cannot buy anything on Thursday after selling because you don't have any money yet. You have to wait 3 days after selling before the money is returned to you.

Case2:
monday - 10k invested on 'A'
Tuesday- sold 'A' completely
Tuesday(same day)- cannot buy 'A' but can buy 10k worth of 'B'
Not correct. Same reason as Case 1.

Case 3:
monday - 10k invested on 'A'
monday- sold 'A' completely
friday- can buy anything
Correct!

comment on correctness of below cases in regard to day trading. (cash account w/10k)

case 4: cannot buy and sell 'A' on same day at all
Not correct. You can buy and then sell just a few seconds later. This is considered a "day trade" and you can only do 3 of those within a 5-day trading period unless you have a margin account with $25,000 or more. Read more here.

case 5: can buy and sell 'A' on same day but cannot rebuy after selling.
It depends if you have money or not. You are allowed to continuously buy and sell as long as you have money (depends on the settlement dates) and you are not violating the 3 day trades in 5 days rule, both of which can be overcome with a margin account with enough cash in it.