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Tuesday, March 07, 2017

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.

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