# The infinity of primes

by kenewbie
Tags: euclid proof primes
 P: 236 Euclid's proof: 1) Assume there is a finite number of primes. 2) Let Pn be the largest prime. 3) Let X be the P1 * P2 ... * Pn + 1 At this point the statement is that "X cannot be divided by P1 through Pn", but why is that? This is not self-obvious to me. How can I know this? k
 Math Emeritus Sci Advisor Thanks PF Gold P: 39,552 Because dividing by any of those primes gives a remainder of 1, not 0: $$\frac{P_1P_2...P_{i-1}P_iP_{i+1}...Pn+ 1}{Pi}= P_1P_2...P_{i-1}P_{i+1}...Pn+ \frac{1}{P_i}$$ The first term is an integer but the second is not.
 Emeritus Sci Advisor PF Gold P: 16,091 Minor aside: "the infinity of primes" is bad grammar. The noun form is wrong here -- you want the adjective "infinite", such as in "the infinite set of primes".
Emeritus
PF Gold
P: 16,091
The infinity of primes

 Quote by kenewbie How can I know this?
Try dividing.
P: 236
 Quote by HallsofIvy $$\frac{P_1P_2...P_{i-1}P_iP_{i+1}...Pn+ 1}{Pi}= P_1P_2...P_{i-1}P_{i+1}...Pn+ \frac{1}{P_i}$$ The first term is an integer but the second is not.
That made perfect sense, thank you!

k

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