hosseinimr93 After seeing your algorithm, I tried to do it in C++, but I don't understand why it's getting a minus. I tried with the help of that source, but the answer was a minus.
I am not familiar with C++, but I feel you are calculating the exp_term wrongly.
You are totally right BlackHatCoiner but those are 2 different things, one thing is the 51% attack, where the miners can modify confirmed blocks, and the second thing is the double spend where the user modifies the unconfirmed transaction.
I think you didn't understand BlackHatCoiner.
Any mining pool can try to perform a 51% attack and you don't have to own more than 51% of the total hash power to do that. That's what BlackHatCoiner says. He didn't say anything about double-spending.