Each draw is an independent event. Buying a single ticket in 6/49 this week gives you a 1 in 14 million chance to win. Next week, buying a single ticket in 6/49 gives you a 1 in 14 million chance to win. Since each event is independent, the odds are not additive. In other words, the odds are not (1 in 14 million + 1 in 14 M = 2 in 14 Million). The odds are not 2 in 14M, they remain at 1 in 14M.
Your chance of winning on the second ticket indeed remains 1/14M. However, your odds of winning on either draw, before the first draw is made, is additive.
I'll write here about lottery strategies, which looks at the odds of different series of ticket buys. This is done prior to any draw, to determine what the expected odds or return is on a strategy as a whole.
The math for the two draw strategy goes like this. The key thing about probability is to remember:
Pr[Event A or B] = Pr[Event A] + Pr[Event B] - Pr[Event A and B].
The last part is like a correction for winning multiple jackpots, which either is not necessary for tickets in the same draw, or ends up being miniscule in over multple draws.
Often you can tackle a problem the other way:
Pr[Event A or B] = 1 - Pr[ not A and not B]
Code:
Probability of winning either week =
= 1 - Probabililty of losing both weeks
= 1 - (1 - 1/14M)(1 - 1/14M)
= 1 - 1 + 2(1/14M) - (1/14M)*(1/14M)
= 2 x 1/14M approximately
(the correction for winning twice is very small)
So, the odds of winning over the two weeks is 2/14M approximately. If you lose the first week, your odds collapse in a sense when going into the second week, down to 1/14M.
So now, let's say two friends each buy a ticket this week for 6/49. Each has a 1 in 14M chance to win. Next week, only one friend buys a ticket. His chance of winning is 1 in 14M, while his buddy who did not play has 0% chance. So the ticket buying friends chances are infinitely better of winning a prize in week two vs his abstaining friend. But over the course of the two draws, the ticket buying friends chance of winning is still just 1 in 14M, same as his friend who only bought a ticket in week one.
Think about it this way. One of them has a strategy to buy one ticket in week one, but abstain in week two. The other has a strategy to buy a ticket each week.
Over the course of the two draws, the odds of their two strategies winning are different. One is 1/14MM, the other is 2/14MM roughly, from the calculation above.
This doesn't mean the odds of the second week's ticket are not 1/14M when viewed in isolation.
Also, recognize that buying 10 tickets this week is different then buying one ticket for 10 weeks. If you buy 10 tix this week, the odds are additive since you are entering several times in the same event. Thus, your odds improve to 10 in 14 million (or 1 in 1.4M). If you play one ticket each week for ten weeks, then you are participating just once in 10 independent events and your odds stay at 1 in 14 million, not just for each draw, but over the course of the entire 10 draws.
The odds of 10 distinct tickets winning the jackpot in a single draw and the odds of 10 tickets over 10 draws are very close to each other.
The math goes like this for the two strategies:
Code:
Probability of a jackpot in 10 tickets in a single draw
= Prob Ticket 1 or 2 or 3 up to 10 win
= Prob(Ticket 1 wins) + Prob(Ticket 2 wins) + ... + Prob(Ticket 10 wins)
(this works because multiple tickets cannot win, so no need to correct for multiple jackpots)
= 10 x 1/14M
Probability of a jackpot in 10 tickets over 10 draws
= 1 - Probability Zero wins in the 10 tickets over 10 draws
= 1 - (1 - 1/14M)^10
= 10 x 1/14M approximately
Again, this doesn't mean the odds of a single ticket in a single draw is not 1/14M. It just means that the strategy of buying 10 distinct tickets in a single draw is very much the same as 10 tickets over 10 draws.