Interesting series

Consider the following series written in form of a decimal 1 9 + 1 99 + 1 999 + 1 9999 . . . + 1 1 0 n 1 + . . . \frac { 1 }{ 9 } +\frac { 1 }{ 99 } +\frac { 1 }{ 999 } +\frac { 1 }{ 9999 } ...+\frac { 1 }{ { 1 }0^{ n }-1 } +...\\

Find the digit in the 2014 s t 2014st place after the decimal point.


The answer is 0.

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1 solution

Antony Diaz
May 3, 2014

First, we have that

1 9 = 0 , 111111... 1 99 = 0 , 010101... 1 999 = 0 , 001001... 1 9999 = 0 , 00010001... . . . . . . \frac { 1 }{ 9 } =0,111111...\\ \frac { 1 }{ 99 } =0,010101...\\ \frac { 1 }{ 999 } =0,001001...\\ \frac { 1 }{ 9999 } =0,00010001...\\ .\\ ..\\ ...\\

So clearly the digit in the place will be equal to the number of factors of that number.

Now to calculate what is the number of factors of 2014 2014 , before we need the prime representation 2014 = 2 x 19 x 53 2014=2x19x53 .

So we use the function d ( 2014 ) = d ( 2 19 53 ) = 8 d(2014)=d(2\cdot 19\cdot 53)=8 .

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N o t e : Note:

If we have a number n = a 1 k 1 a 2 k 2 a 3 k 3 . . . a n n n n={ { a }_{ 1 } }^{ { k }_{ 1 } }{ { a }_{ 2 } }^{ { k }_{ 2 } }{ { a }_{ 3 } }^{ { k }_{ 3 } }...{ { a }_{ n } }^{ { n }_{ n } } , the number of factor of n is ( k 1 + 1 ) ( k 2 + 1 ) ( k 3 + 1 ) . . . ( k n + 1 ) ({ k }_{ 1 }+1)({ k }_{ 2 }+1)({ k }_{ 3 }+1)...({ k }_{ n }+1) .

There are carry overs from 2015th and 2016th decimal places..

Vishal Yadav - 4 years, 2 months ago

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