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An infi…nite sequence of positive integers is called a

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An infi…nite sequence of positive integers is called a “coprime sequence ”if no term in the sequence shares a common divisor (except 1) with any other term in the sequence. If S is an in…finite sequence of distinct positive integers, is S a coprime sequence?

(1) An infinite number of integers in S are prime.
(2) Each term in S has exactly two factors.


[Reveal] Spoiler:
Struggling to understand the answer as B.

Considering statement 1

If integers in S are prime they have exactly two factors and therefore this statement is same as B and therefore should be sufficient.

Where I am not getting this right guys?
[Reveal] Spoiler: OA

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An infi…nite sequence of positive integers is called a “coprime sequence ”if no term in the sequence shares a common divisor (except 1) with any other term in the sequence. If S is an in…finite sequence of distinct positive integers, is S a coprime sequence?

Notice that S is an in…finite sequence of distinct positive integers.

(1) An infinite number of integers in S are prime --> obviously these primes will be coprime to each other. But we don't know whether the sequence contains some numbers other than primes, and if it does then the sequence won't be coprime (for example the sequence can contain 4 and 6 in addition to these primes). Not Sufficient.

(2) Each term in S has exactly two factors --> each term in S is a prime, so S contains only distinct primes, which will be coprime. Sufficient.

Answer: B.
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New post 08 Feb 2014, 03:38
Nice, nice nice. Fell into trap and chose wrong answer. Statement 1 seems like each number in s is prime but it is not! Statement 2 makes it look like the numbers are not prime but indeed they are!
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New post 16 Apr 2015, 17:53
Bunuel: if two terms are 2 in the sequence S....(2,2,3,4,5..............) here the two terms have 2 as a common divisor. The question says that a coprime sequence will not have any other factor common to any other number except 1.
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sayansarkar wrote:
Bunuel: if two terms are 2 in the sequence S....(2,2,3,4,5..............) here the two terms have 2 as a common divisor. The question says that a coprime sequence will not have any other factor common to any other number except 1.


You are given that S has distinct integers. So two terms cannot be 2 each.
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New post 17 Apr 2015, 00:57
Thanks Karishma....You have discovered my other problem....I tend to read super fast and miss things in the process....my biggest weakness...
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New post 17 Apr 2015, 01:03
sayansarkar wrote:
Thanks Karishma....You have discovered my other problem....I tend to read super fast and miss things in the process....my biggest weakness...


Reading super fast is great for RC. For all others, it is not advisable. You have enough time so slow down a bit.
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Re: An infi…nite sequence of positive integers is called a [#permalink]

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New post 09 Jun 2016, 15:12
Bunuel wrote:
An infi…nite sequence of positive integers is called a “coprime sequence ”if no term in the sequence shares a common divisor (except 1) with any other term in the sequence. If S is an in…finite sequence of distinct positive integers, is S a coprime sequence?

Notice that S is an in…finite sequence of distinct positive integers.

(1) An infinite number of integers in S are prime --> obviously these primes will be coprime to each other. But we don't know whether the sequence contains some numbers other than primes, and if it does then the sequence won't be coprime (for example the sequence can contain 4 and 6 in addition to these primes). Not Sufficient.

(2) Each term in S has exactly two factors --> each term in S is a prime, so S contains only distinct primes, which will be coprime. Sufficient.

Answer: B.


Great analysis.

Can you just provide similar tricky(referred to Statement 1 type trap) question to practice?

Thanks
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