Consider the set of natural numbers N . Divide this set into two subsets, S 1 and S 2 , where { S 1 ∪ S 2 } = N and { S 1 ∩ S 2 } = ∅ . Is it possible to construct S 1 and S 2 such that:
1) Neither S 1 nor S 2 contains any arithmetic progressions of any common difference and of infinite length
2) Neither S 1 nor S 2 contains any geometric progressions of any common ratio and of infinite length?
For clarification, a progression of finite length is allowed. For example, { 1 , 3 , 9 , 1 8 . . . } , which contains a geometric progression with common ratio 3 and length 3, is allowed. However { 2 n ∀ n ∈ N } = { 2 , 4 , 6 , . . . } , which contains an arithmetic progression with common difference 2 and infinite length, is not allowed.
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Disclaimer: This is by no means a rigorous proof but just an intuition on which a proof can be built upon.
The first condition can be satisfied by having 'gaps' in both S 1 and S 2 that grow faster in size than linearly. An example of such a construction is having S 1 be the set of integers whose number of digits is odd and S 2 be the set of integers whose number of digits is even. The intuition is that for any arithmetic progression a n = a 0 + k n , where k is the common difference, there would exist an n ′ such that for all n > n ′ , the gap is larger than k and so one of a n is going to have to fall through the gap and belong to the other set.
That said, the 2nd condition can be seen similarly by noticing that for any geometric progression g n , the number of prime factors counted with multiplicity follows an arithmetic progression. A construction that disallows such would satisfy the 2nd condition.
Hence the construction below satisfies both conditions:
S 1 is the set of integers whose number of prime factors, counted with multiplicity, has an even number of digits. S 2 is the set of integers whose number of prime factors, counted with multiplicity, has an odd number of digits.
Not sure but S 1 can be assigned any random natural numbers that follow the given description, which is possible because I can design an example, and S 2 = n ϵ N − S 1
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S 1 = { n ∈ N : if ⌊ lo g 1 0 ( 1 + lo g 1 0 n ) ⌋ even }
S 2 = N ∖ S 1