Harmonic series without number 9 is convergent
There are tons of proofs that harmonic series diverges. What if we have a harmonic series where we get rid of all the numbers which contains 9, will it still converge.
For single digit there are 8 numbers which do not contain number 9.
Single digit numbers 8
Two digit numbers 8 \times 9
Three digit numbers 8 \times 9 \times 9 = 8\cdot 9^2
4 digit numbers latex 8\cdot 9^3
...
Also notice that largest fraction in harmonic series
Two digits \frac{1}{10}
Three digits \frac{1}{100}
Four digits \frac{1}{1000}
So if we replace the terms of harmonic with these fractions we will get an upper bound on the harmonic series devoid of number 9
It is \frac{1}{1}\cdot 8+\frac{1}{10}\cdot 8 \cdot 9+\frac{1}{100}\cdot 8 \cdot 9^2+\cdots+.. and
\Rightarrow 8\cdot \frac{1}{1}+8\cdot \frac{9}{10}+8\cdot \frac{9^2}{10^2}+8\cdot \frac{9^3}{10^3}+\cdots
This is a geometric series and the sum is
\frac{8}{1-\frac{9}{10}} = 80.
Thus the geometric series with common ratio of \frac{9}{10} is convergent.
Labels: 9, Convergence, Harmonic series
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