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Famous Theorems of Mathematics/Sum of the reciprocals of the primes diverges
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The series
diverges, where is the nth prime number. This proof is a proof by contradiction.
Proof
Assume for the sake of contradiction that
converges. We pick a large enough number to make a number such that
We create a new geometric series
This geometric series converges because Expanding this series yields
When expanding each term, this series will contain all fractions of where only consists of prime factors in the set although there will be many duplicates of this fraction. Therefore,
and thus this series converges.
Consider a new series
The denominator of each term in this series, , must only have prime factors in the set This is because if it did have a prime factor less than , say , then is an integer and thus is an integer, but this is impossible.
So, must only have prime factors in the set which means
and thus this new series converges. However,
This would mean that also converges, but this is just the harmonic series, which diverges. A contradiction. ∎